If Taliban starts it's new regime with bloodshed they lose the war of perception. If they kill fleeing civilians they can't exactly sell themselves as peaceful crusaders fighting against evil America.
In Afghanistan there is one rule, the rule of the strongest.
The Taliban is simply the strongest tribe and thus in power.
If they show weakness the Northern Alliance or ISIS-K will take-over.
When Abu Ivanka al-Amriki was ousted the US showed weakness and the Taliban rolled over them.
That's how it has always been in Afghanistan, people side with the strongest faction.
They've already killed quite a number of people outside of Kabul.
If the Taliban was no real threat women wouldn't throw their babies over barbed wire.
The first thing they did when they took Kandahar is arrest, humiliate and kill a famous local comedian.
