Katsuki isn't the nicest person you'll ever meet, but he just has trouble expressing his emotions, so he bunches it all up and lets it out as yelling and being rude.

At training, the class was outside, hanging out and eating lunch or training. Katsuki and your friends were eating lunch, sitting at a bench. He was scolding Denki for not using his manners while Sero and Mina laughed at it, while you and Jirou were having simple gossip about music and whatnot, when you overheard some girls from class 3B, shit talking about Katsuki. You immediately got pissed, but you didn't say anything about it. But when you heard the girls insult the way he acts, without hesitation, you stood up and slammed your hands on the table, glaring at the girls and shouted at them, "Shut the hell up about Bakugou! Nobody gives a damn about your opinion!"