Why does Ender's older brother beat his little brother up? Why does he hate him so much?

Peter has the makings of a sociopath. He constantly bullies and belittles his brother Ender. Peter is jealous when Ender is singled out to join the prestigious Battle School and he is not.

Peter's just jealous that's all.

I don't know why but I guess he's jealous that Ender is singled out to join the prestigious Battle School while Peter is stuck at home . . . I would be exactly like Peter, "Great, now I can't go to Battle School because my little brother is better than me than I am at him!" I would be like him too.

As you may know, Ender is a "third". Peter uses this as an excuse to take all of his anger out on something. He believes Ender never should have existed.