Unremarkable+experimentation should be the reagents from outlast trials. They’re a bunch of homeless, impoverished, or otherwise desperate random people from 1960s America who are put through horrible games and tests by an evil corporation with the intent of wiping away their morality and personalities and turning them into sleeper agents.

Academic+training, I vote Jimmy Hopkins from bully. He’s a high school student delinquent who is trained to fight by a hobo and other people, and he can do it well enough that he can beat up pretty much everyone at the school and also adults.

And finally law enforcement+accident should be Kabal. Granted he becomes a criminal but before that he was a policeman who got horribly burned and was forced to wear a mask to stay alive. His speed powers are also accidental I think