The final episode of this fascinating series asked how an awkward, people-pleasing doctor turned into a dictator who went to war against his people

The third and final instalment of A Dangerous Dynasty: House of Assad (BBC Two) began with an interview with a Syrian doctor. Zaher Sahloul went to medical school with the Syrian president Bashar al-Assad and was acquainted with him in those days. “It’s very ironic,” he said. “We’re classmates and one of us is bombing hospitals. And one of us is treating the victims of the bombing.”

That comment sums up the question this fascinating and horrifying series sought to answer. How did a shy, awkward, people-pleasing ophthalmologist become a murderous tyrant? How did someone trained to heal the sick end up killing more than half a million Syrians?

His wife is no less perplexing a figure: born in the UK and raised in Acton, west London, Asma al-Assad was about to go to Harvard to study for an master’s when she chose instead to marry Assad, who had inherited his rule from his father after his older brother was killed in a car accident. Glamorous and articulate, the new first lady initially appeared to be a reform-minded influence. She was also keen to promote her dictator husband’s cuddly side. “Probably one of the best things about him is that he’s so easy to talk to,” she once said. In light of events, her most anodyne words now sound ominous.

Part three covered the roots of the present war. Asma was being interviewed for a spectacularly ill-timed Vogue profile on the day in late 2010 that a fruit vendor called Mohamed Bouazizi set himself on fire in Tunisia to protest against government corruption, an incident that would kickstart the Arab spring. By the time the Vogue piece hit the news stands (labelling Asma “the Desert Rose”), waves of unrest were lapping at Syria’s borders. In Deraa, some children plastered the front of their school with graffiti – “Your turn next, doctor”, it said. The first demonstrations began in response to their arrest and torture.

In those early days, there was some hope that Assad might avoid disaster by opting for reform instead of a crackdown. “The wild horse of revolution has arrived in Deraa without a knight,” one of Assad’s ministers wrote to him. “I would hope that you would be that knight.” But he opted not only for repression, but for stunning brutality.

Assad is such an unlikely monster that it is hard to believe he is wholly in charge. Intelligence showed that his mother consistently pressured him to take a tough line and it is possible that some atrocities may have been outsourced to his younger brother, Maher. But the idea of Assad as the mild-mannered face of a brutal regime controlled by other elements of the family is misleading. Paperwork smuggled out of the country shows that he knew about – and signed off on – everything.

A carefully threaded mix of archive footage and contemporary interviews, A Dangerous Dynasty has retained an eye for details throughout that are both absurd and chilling: male Syrian army recruits were required to stab puppies in front of Assad’s father, Hafez, to demonstrate their loyalty; female recruits had to bite the heads off snakes.

As Bashar went to war against his people, the scale of his crimes began to outstrip the documentary’s ability to record them. Protesters were killed in their hundreds. Whole neighbourhoods were destroyed by government shelling. One moral line was crossed after another. The journalist Marie Colvin was targeted, using her mobile phone signal, and killed. The use of sarin gas made it clear the regime was capable of anything. A Dangerous Dynasty was unflinching. I flinched repeatedly.

Again, the figure of Asma presented an enigma. Certainly Assad failed his test of humanity, but what of her? Could she countenance the regime’s actions? Did she have a choice? Emails obtained in 2012 by the Guardian show that she was, in fact, ordering a lot of stuff from Harrods. Whenever she emerged in public, her declarations of loyalty were tireless.

Finally, A Dangerous Dynasty reminded us that Assad’s decision to plunge Syria into civil war was a massive miscalculation: he was losing. At one point he controlled less than 20% of the country. Had Russia not come to his rescue, he would be gone.

If you missed the first two instalments, it is worth going back for a thoroughly terrifying lesson in how we ended up here.