Katsuki has many regrets, too proud to voice any of them.
That doesn't stop him from constantly thinking about them though, slowly driving himself to insanity.
Katsuki Bakugou hates a lot of things. Those things including Izuku Midoriya, who is at the very top of the list. So he doesn't know why the fuck he kisses him. Or why the hell he gets so jealous seeing him with someone else. It's not like he can control it. If only that idiot didn't just let it happen. Then maybe what happened wouldn't have.