An incarcerated terrorist leader is more useful than a dead one. Information, or at worst they could go for a war of the best ala one piece by announcing an execution.

Izuku wanted to respond to Bakugo's strong feelings, but he didn't forget his responsibility to his own body that has been drilled into him. Less than 10 episodes ago he said he would never again use 100 percent (lol, but for now he's kept that promise).

As for pinned down vs strength, the point was that Bakugo had him in a kill state. That posture, hand on face, is little different than holding a gun to your face. Unless you trained up your hard face suffeciently, he was being held against death; hence Bakugo won. You've seen this before in anime: sword to throat to show who "won" without anyone actually dying. Would you be as upset if it was sigaraki's fingers on his neck like the end of season 2? Why didn't his super strength save him then? Because his life was in someone else's hands, not because he was beat to a pulp. Super strength can't just counter everything.