Kablog night evenly

In the begging of the book this boy is on this train with his father and other JEWS and his father has a piece of bread size of a quarter and everybody is hungry so the kid goes up and ask his father if he could have a piece but the father said no so the boy was forced to kill his father if he wanted to survive.  My reaction is that is sad but wow just wow killing your own father for a piece of bread( that`s pretty harsh to have some food).  My question is why cant he knock him out instead of killing him?