Cook Vs. [spoiler]

I thought Cook and Martin Bishop were (sort of....) friends for a while. I understand Cook thinking Martin turned on him, but Martin was only trying to help starving people when he allowed them to steal from Cook's supplies. (And they weren't even Cook's supplies, they belonged to the camp.) And when the group trapped Cook in the outhouse, they only did so in order to survive. It was not out of spite.

So at the end of the movie, when Cook talks about Martin flying away (suggesting he jump to his death) and then sitting and waiting with a gun until Martin falls to his death...it did not make sense to me that Cook became this murderous thug.

Lots of confusing things about people in this movie. Was every single person insane other than Martin???