Ever since he was little he was praised by everyone for having an awesome quirk. His mom tried to curb that, yet everyone else was spoiling him.
He’s also a bully. So when Deku started actually sticking up for himself and actually developing a quirk, it was like your favorite punching bag started to punch back.
When he got to UA he wasn’t treated with admiration. He didn’t have the pedestal he thought he deserved. So he lashes out more & more. Yet at same time he’s firmly dedicated to being a hero.