He of course has plenty of reason to want to kill, and he would be justified to do so, but I think a lot of normal people would have a hard time to actually do so under most circumstances...especially a 19 year old.
He doesn't have laser eyes or gun fingers. Any killing he does is going to be with his bare hands. We saw how brutal that has been so far. That's way different from shooting someone. A lot easier to detach yourself from what you're doing when you have some distance.
18 year olds in history were in a war, they were training to kill. Mark is specifically against killing, so the mindset isn't the same. His goal is to save people, not kill the bad guy.
I'm not saying that his dad killing innocent people is the same as him killing a bad guy that deserves it--but, he obviously cares about his image, how other people will see him, and about where he could wind up. He doesn't want to test that line.
He just saw 18 other versions of him that were all evil---didn't care about human life. And all of the Viltrimites are telling him how soft he is, so that's part of who he is.
To be clear, I don't think either way is wrong. If he starts killing bad guys now, it'll makes sense. But I think the show has established a lot of good reasons why it is something he doesn't want to do.