Was watching Batman Begins while noodling around the house just now when this question came to mind. By mainstream, I mean a hero that has caché outside the world of comic fandom. That precedes MCU as lots of people grew up with cartoons and video games but who never got into comics. So out of the heroes that are broadly and popularly recognized, who's the meanest?
I mean, it's gotta be Batman, right? He doesn't kill, but he leaves a lot of broken bones and busted organs in his wake. The brain trauma alone he has caused people is criminal, even when inflicted on criminals.