Katsuki doesn't leave Deku's vicinity or utter those words without half expecting him to try to stop him--he just hopes that the moron has the presence of mind to let Katsuki save both of their asses from complete and utter humiliation??? Either way, that expectation is why Midoriya doesn't succeed in actually tackling him when his weight slams into Katsuki's back, eliciting a grunt from the explosive teen on impact. They end up veering into the wall instead, another hard thump as the blond snarls in frustration yet again. Twisting his body in his grip to allow for the right angle to do so, Bakugou's elbow is swinging back and down to hit him wherever the fuck he can with it. Face, the top of his head, that exact location isn't the important part. Just that he's getting nailed, hard.

"Let go of me before I kill you, you shitty nerd! Ain't like it's gonna fucking hurt em!" That part is loud, but thankfully? Your blond friend ain't totally stupid, and the next part gets hissed out much quieter through clenched teeth. All the while, the hand that doesn't belong to the elbow that is certainly going to continue landing blows? Moves to try to tear apart Deku's grip around his center.

Don't make him get serious about this struggle, dude. Katsuki is about two fucking seconds from that. "No way in hell am I gonna let that jackass seeing my fuckin' face and think we're doing anything like that in here! The hell is wrong with you?!"