THere are several instances where the boy can sense it. Maybe it's part of the "passing" on the Fathers morals, goals, whatever, that the Boy is more anxious about becoming a bad guy, and he can see it happening in his father as he grows more and more desperate.
Yea that scene bothered me a lot yea. I think the father was losing his humanity & didn't care about anything or anyone but his son by that point u know.
In that environment, a person who steals your food might as well be trying to kill you. I have no sympathy for the thief. He deserved to have all his stuff taken. Maybe he even deserved to be killed.