The guy forgot his goal, his comrades sacrifice and support of them?
Well he was about 11 years old, but he should have known better by now instead locking himself out in the shelter, it's not what they wished for.
Thought there was some sort of struggle and he killed them all, judging by how he has no hesitation now to kill even one of them just to get rid of the rest.
This is kinda weird to be honest...