The war in the eastern Democratic Republic of Congo has been the most faceless of conflicts. The unbearable statistics threaten to fade into abstraction: an estimated four million people killed, tens of thousands of women raped, countless numbers displaced and lives destroyed. But last week, for me and many others, the bloodiest and least reported horror story of our times took on the indelible features of a single face.

At 46, Eraste Rwatangabo was a few months older than me, but he looked far younger. I met him in October 2008 at his home on the high plateau of South Kivu in the far east of the Democratic Republic of Congo (DRC). No one who has been to that stark corner of the world forgets its atmosphere. It is like an otherwordly Salisbury Plain rising out of the dense jungle that flanks it on all sides. Every morning a thick mist hangs over the plateau, which stretches, gently undulating, for hundreds of miles. And almost every morning, for more years than anyone can care to count, out of that soundless mist have come new stories of displacement and terror, sometimes three days' walk away, sometimes in the next village.

The high plateau, rich in mineral resources, including gold, has been the battleground in a ceaseless conflict that over nearly two decades has drawn in six countries; it has suffered both the brutal fallout of the genocide in neighbouring Rwanda and the chaotic ethnic and factional war in which the lawless Congolese army has been only one of many actors (militias formed mostly of Ugandan and Rwandan refugees have come and gone; local "defence forces" and "rebel units" remain).

It is a few hundred miles upstream from Conrad's Heart of Darkness. The people who live here, predominantly the pastoralist Banyamulenge, have known little but relentless hardship and sporadic violence. Many of them, not surprisingly, have faces etched in anxiety and look far older than their years.

Rwatangabo immediately stood out. Educated to degree level in history and articulate in several languages, he carried the idea of a different future for this plateau in his easy stride and broad smile. For the best part of a week, travelling in his company on the high plateau, there seemed nothing or no one that he did not know; we met no adult who did not hang on his words, no child who did not want to hold his hand. The local people called him "captain", but his mission had never been a military one. Rwatangabo was a fighter for hope, and for education, in a place where there was precious little of either.

He had grown up in South Kivu, married in 1982, and had eight children, two of whom had died. After completing his degree in Rwanda he returned to the high plateau to teach and became a schools inspector before working for the Red Cross. In 2006, during the country's first national elections, he was the president of the election commission for the region, travelling between villages explaining to people why they must vote.

Since then, in the most desperate corner of the most ungovernable country on Earth, he had been responsible for wielding perhaps the most powerful of all weapons for change: a teacher training programme. This was no ordinary course of study. Established by the Rev Samson Muvunyi, of a Christian mission called Eben-Ezer Ministries (EMI), it had grown out of a peace and reconciliation movement that sought to include all the factions divided by war; to give children a textbook rather than a machete to carry; to build a fresh start.

In 2008, I had flown to South Kivu to report on an emblematic story of that fresh start; one related to a part of the tragedy that had reached close to home. Ron and Pauline Friend, from Orpington, Kent, had sponsored a school in memory of their son, who had been caught up in the conflict in 1999 while travelling on a gap year. Martin Friend had been murdered by former Interahamwe génocidaires from Rwanda, and Ron and Pauline were travelling to the high plateau to open the school that they had raised the money to pay for in their son's name. It was a profoundly emotional journey. The school had been built in a place, Mishashu, where there had previously been not a stick of chalk. It was constructed by the women of the village, who had hauled stone and cement and water up miles from the road. At its opening, the 300 children who would study at the school couldn't stop wondering at the marvel of desks and whitewashed walls.

The more time we spent on the high plateau, though, the more we realised that Ron and Pauline's school wasn't an isolated miracle, but part of a growing network. That network was being established and strengthened by the British charity Children in Crisis in partnership with the EMI programme led by Muvunyi and Rwatangabo. A total of 192 schools over a vast area were included in this programme. Many had been burned or destroyed in the conflict. But nearly all, it seemed – with the efforts of EMI and Children in Crisis – were being restored as brick-built symbols of possibility. A sense of one-upmanship and competition had even been established. Parents, just as sharp-elbowed as any in north London, were placing their children with relatives to sneak into a decent catchment area.

This was made possible in large part by Rwatangabo's energy in inspiring his team to plan schools and establish training programmes and adult education that work to break down the distrust and division that was a legacy of war, even as the war in one form or another was still being waged. It was a mission to prove that even in this place there might be another way.

Last week, however, the chaotic violence that Rwatangabo had been born into, and which he had done so much to fight against, finally caught up with him. On the long road through the jungle that leads up from Uvira, near the border with Burundi, to the high plateau – a 24-hour drive that represented the teacher trainers' journey to work – Rwatangabo was ambushed by a militia group and murdered at the side of the road, along with four colleagues and two of their relatives.

They had been on their way to do a one-month programme with six teachers and one headteacher from 18 of the more remote schools in the Fizi territory on the plateau. The teachers they were to meet were on the second stage of the intensive study initiative that involved the setting of targets and the sharing of best practice. The programme had used, very effectively, theatre workshops to train participants in sketches and role plays around issues of violence and reconciliation and the place of women. The leader of that theatre programme, Pastor Antoine Munyiginya, was also on board.

They had left Uvira on 4 October, 13 people packed in a Land Cruiser. It had not stopped raining. Along a road that was essentially a loosely linked track of craters and rocks they were further hampered by flooding. They had been pressing on to catch up with a Red Cross vehicle that was two hours ahead, so that they could help in pulling each other out of the worst of the mud. By about five in the afternoon of a long day, they were 18km from the village of Baraka where the Red Cross vehicle was waiting.

It was there that they were ambushed by a Mai Mai Yakatumba group. Mai Mai is the name given to any of the shifting and desperate gangs who get hold of some semi-automatic weapons and call themselves a rebel militia. Yakatumba is the name of one of the more notorious local warlords.

As I briefly experienced in my time with Rwatangabo and his team, at dusk around every bend of the slowest road imaginable you could easily meet a group of young men, perhaps drunk and armed. Mostly they could be bought off with some cigarettes or a small amount of money. But you never knew.

The Mai Mai that attacked the Land Cruiser had, apparently, only one aim, however. They wanted to kill any Banyamulenge on board, members of the Tutsi-related ethnic group whom the "indigenous" Congolese Mai Mai consider incomers (though Banyamulenge have inhabited the plateau for centuries). Stopping the car, they ordered "the Rwandans" to get out of the vehicle. Two men and two women of different ethnicity who were travelling with Rwatangabo and his team were told to leave.

The account of what happened next comes from Muvunyi, who spoke to Munyiginya, who survived the attack. The driver of the vehicle, Musore Ruturutsa, who had worked with Eben-Ezer for three years, was shot in the back. The others were taken a little way into the jungle. They were being led by Munyiginya in songs and prayers. Each one, systematically – Rwatangabo; Kandoti Tite, his deputy education manager; Gifota Edmond, a newly recruited teacher trainer; Nabisage Rganza, 24, the sister of another colleague; and two others – was murdered in turn.

Eventually only Munyiginya was left. He was told to lie face down and the Mai Mai commander gave the order to "finish the job". A gun was put to his head, but he was shot in the hand. Antoine did not know why he was spared. Perhaps to tell the tale. He is in hospital with a bullet still lodged in his wrist and there is no doctor in the area capable of removing it.

The seven people murdered were buried two days later. In a place where any journey takes on the quality of an epic quest, 2,000 people attended the funeral at a day's notice. The president's wife, who had been in the region, joined the mourning. A memorial service will be held next Sunday in Uvira, at which thousands more are expected.

Sarah Rowse, project director for Children in Crisis, who has spent many months on the high plateau over the past five years, helping to establish the programme, will travel out for the service along with two of her colleagues. Last week, she was still struggling to comprehend the news. "There are probably contexts in which you can try to understand it," she said, "but the thing about Eraste was that he always just seemed bulletproof. He and Samson helped to make everyone feel a bit like that; they always seemed to know what was going to happen before it happened."

The contexts include the second national elections, which are scheduled for the end of November. President Joseph Kabila, whose family ties are in the Kivu area, has made the (extremely relative) stability of the region in recent years the main platform of his campaign. "No more fires in the east," he said "only embers". In this sense the murders could look like a premeditated effort to give the lie to that slogan.

Muvunyi suggested that it was inconceivable that the Mai Mai did not know the work of EMI, and who they were killing. He and Rwatangabo had spent years negotiating with various militias to explain their mission. As Rowse said, "Eraste worked tirelessly talking to commanders saying let these teachers travel in peace, [urging them] to not make the children carry munitions or weapons as they tended to do. People have listened to him. That's one of the reasons it just seems so unbelievable. Why it is a pure hate crime."

Many are seeing the killings as further proof of genocide against the Banyamulenge. A childhood friend of Rwatangabo, Alex Ntung, now lives in Hastings, East Sussex. Ntung lost members of his family in one of the most infamous massacres of the war, at the Gatumba refugee camp in 2004. He has fought since for the atrocities to be recognised as genocide by the UN. The murder of his friends, he told me, is more evidence of the intention of "killers to exterminate a group of people, selectively and sophisticatedly".

At the very least the murders must pose hard questions of the role of the international community in the area. The UN has long had a large and extremely well-resourced "peacekeeping" force, formerly Monuc, now Monesco, currently numbering 17,000 men, in this region of the DRC. The remit of this force has never been clear, but guaranteeing the safe passage of organisations like EMI, which offer the best hope of that peace, has never been part of it. If western aid workers want to get to the high plateau from Uvira they can often find a seat on the UN helicopter which makes a daily flight (as we did in 2008). Those, like Eraste Rwatangabo, who are closest to the problems, and the solutions, have never, for whatever reason, been afforded that security.

Despite their anger at the loss, Rowse and Muvunyi are determined that the education programme will continue. The immediate concern, they say, is to try to look after the families of those who were murdered and who have no means of support. A fund has been established. After that, the programme must find a way to continue. "Although Eraste's life has been cut short, and those of his Tite and Musore and Edmond, what they have done can't be lost," Rowse said. "It's just too dreadful to contemplate that their lives will have gone in vain. We must continue to believe that the foundations for peace are there in those schools and those teachers."

For more information or to make a donation to support the families of those killed, go to childrenincrisis.org/drcappeal