Being a villain in the league, you understood AFO's power to an extent, so when you saw him lying on the ground, you knew even if they took him into custody, he would get out.

All Might was already half dead on his feet. He would definitely be retiring, and if he retired, there was no way AFO would be defeated again.

After a few months of working under AFO, it didn't take a genius to understand he wasn't fighting with Stain's principles and now was the chance for you to rebel. You were a well known vigilante and the ruined battle ground was crawling with heroes that had all been previously after you so you were well aware you were risking you freedom for this but you couldn't just stand by. You had to kill him.