I can easily comprehend it, the history books are full of people killing large numbers of their own people. They just find some irrelevant differentiating factor that allows them to label the other as the outgroup and bring out the guns, the tanks, the ovens and the bombs.

Also, they know the alternative is that they will be dragged in the streets and killed. Iran is long past the point where a revolution can be peaceful and conciliatory; if the regime falls, there will be a redde rationem where most people connected to enforcement and decision-making will be very summarily judged by the people they abused for decades.