>>713886057
You can still kill civilians without your team betraying you, but it has to look like an accident. No up close executions of kneeling people. That said, you absolutely can lock your team in a room with a wedge and do whatever you like in a mission. You can even finish it and get to the rating screen.

I also downloaded an invincibility mod and had fun fighting my team. They can take a lot of damage, but you can blow all their limbs off with a shotty to put them down.