Why do you suppose Darry acts the way he does? the outsiders

We think he's so hard on Pony because a) he sees Pony's potential and is afraid that if he isn't hard on Pony, then Pony won't reach that potential; and b) he has no experience as a parent! Darry's stepped into this role without any preparation, and is probably really afraid to fail. His little brother's safety is all in his hands, and there's danger lurking around every corner. When Pony doesn't take care of himself or act responsibly, Darry feels frustrated and angry. He has pressure from all sides, and is always on the verge of exploding. According to Pony, Darry wasn't like this before their parents died.

He acts mean I guess if that is what you are reffering to because he is trying to protect his family and he wants to act tough in front of them so they have someone to look up to