Once revered as a pioneer, the Scottish explorer Angus McMillan is now known as “the butcher of Gippsland”.

This reversal of reputation – from virtuous Presbyterian to cold-blooded killer – is the work not just of the people he wronged but of his own relations and the descendants of his closest friends.

In July 1843 at Warrigal Creek, McMillan and his Highland Brigade surrounded a large group of Gunaikurnai people and mercilessly shot between 60 and 150 men, women and children.

The killings were a reprisal for the murder of Ronald Macalister, who was ambushed by a group of Gunaikurnai men he had chased out of his shop hours earlier.

Angus McMillan sitting with two Aboriginal men described as ‘friends’ but who are likely to be massacre survivors. This image was used on a postcard in the early 1900s. Photograph: State Library of Victoria

McMillan led five more massacres after that, at Nuntin, Boney Point, Maffra, Skull Creek and Butchers Creek, but for more than a century they were forgotten by the Europeans, and he was revered as Gippsland’s heroic founding father and “discoverer”.

Word of his great deeds even reached his native Scotland and were proudly displayed in an archive centre. That’s where his great-great-niece, Cal Flyn, first became aware of him.

Flyn, a British writer, and her mother were on a road trip on the Isle of Skye when they ducked into the archive centre to escape the pouring rain. There, in a display of notable Scottish diaspora, she saw his photo.

I can’t change what happened but it’s what we do next that matters Viki Sinclair

“There was a picture of Angus McMillan looking very stern, and a map of Gippsland which I found just enchanting,” Flyn says. “As I was looking at it, my mum peered over my shoulder and said, ‘Oh, we’re related to that guy.’ I was surprised and kind of excited, because I didn’t know we had an explorer in the family.”

Flyn googled McMillan, intending to visit the areas he had explored and write a travel article about them.

“I came across some coverage of how he had been accused of leading massacres in Gippsland,” Flyn says. “There was shock, but not disbelief.

“I think we understand a lot of very dark work went on in the name of colonialism but, certainly, it was very different to understand it in the personal, being related to somebody.”

Viki Sinclair experienced similar horror when she learned that her great-great-grandfather Colin McLaren was one of McMillan’s right-hand men.

From her home in Mirboo North – much closer to the scene of his crimes – Sinclair had watched The Secret River, the ABC television series based on Kate Grenville’s novel of the same name, and wondered.

Cal Flyn, author of Thicker than Water and great-great-niece of Angus McMillan, the butcher of Gippsland

“I’d seen McMillan’s name in relation to Warrigal Creek and I’d had a few thoughts but I put them away because I just didn’t want to go there,” she says.

Then she discovered her great-great-grandfather had travelled across Gippsland with the Highland Brigade. “I was terribly upset when I found out. I felt ashamed and guilty. You feel like you own it – that’s your history, it’s your background.”

They didn’t know it at the time, but Flyn and Sinclair were on similar journeys of discovery.

I don’t think 150 were killed – I hope there were – but I think that’s exaggerated Max Macalister

Flyn travelled to Gippsland. She read McMillan’s letters and diaries, and other historical records, and spoke to members of the Gunaikurnai Land and Waters Aboriginal Corporation.

Out of that experience came Thicker than Water, a mixture of memoir, travel diary and historical account, and Flyn’s attempt to come to terms with being related to a massacre perpetrator.

Sinclair found the book very helpful. “It’s really difficult, you start to feel like it’s who you are and it takes a long time to get through that,” she says. “Cal’s book was very restorative and she was a young woman without an Australian history who came in here fresh and with open eyes.”

Max Macalister’s relatives include Lachlan Macalister, who took part in massacres and who Max has had immortalised in the form of a bust. Photograph: Ciaran O'Mahony/The Guardian

Other descendants of the Highland Brigade are far less introspective.

Max Macalister is descended from Ronald Macalister, the man slain by Gunaikurnai men, and his other relatives – Colin, Thomas and Lachlan Macalister – took part in the reprisals.

“I wouldn’t call it a massacre, I’d call it retribution,” Macalister says. “It was a common event during that period.

“Does that mean I don’t give a fuck about massacres? Well, it depends on the circumstances. If they deserve it, right fucking whack.”

He says his relatives were threatened by Aboriginal people. “If I had a gun I’d have shot them,” he says. “I don’t think 150 were killed – I hope there were – but I think that’s exaggerated.”

Macalister continues to trace his Scottish antecedents. His house is cluttered with memorabilia, family trees and old portraits.

Sinclair, on the other hand, has done her best to contribute to reconciliation. “I can’t change what happened but it’s what we do next that matters,” she says.

Flyn feels the same. “I don’t want to be too loud a voice on this issue because it’s not my place. Truth telling and memorialisation needs to be led by Aboriginal people.”