Katsuki as always hated you after they were little kids since you used to be quirkless but once UA reached you somehow got in because All Might gave you his quirk, OFA. You've been getting stronger and stronger to the point its obvious your more powerful than Katsuki. Katsuki still gets jealous, but he's learned to except that.

Izuku had killed Shigaraki when he was supposed to save Katsuki and he got accused of homicide. This had made Izuku get convinced that it was true, so he went into depression

Izuku have had a whole mental break down during a villain fight, it got so bad it put a horrible reputation on your name, you walk out of the villain scene and see the whole crowd yell at you and say "Your a monster!" and "Your a goddamn villain" and "Get that kid out of here!", it panicked you and Katsuki watched the whole thing from behind, watching everyone call you a monster.

It sickened him...

Katsuki: Why is this happening...

You were currently panicking and hurting inside. Because of your mental break down during a villain fight, killing 12 people including Shigaraki, of course you were being called a monster, but you weren't one.. You were just struggling, being accused of homicide.

Izuku falls to the ground, sitting down and crawling away from the crowds. Izuku was panicked, upset, over stimulated and hurt

Katsuki's eyes widened when watched you fall and crawl away

Katsuki: DAMN IT! EVERYONE SHUT THE HELL UP! HE'S NOT A DAMN MONSTER!

He ran over to you, kneeled down behind you and pulled you in between his legs, his arms around your waist protectively as he glared up at everyone and yelled at whoever dared saying anything to you to hurt you more, or if someone tired getting close, he knows your hurting and pained, but if you snap.. Your mental health could break.