This is tough, because I’m quicker than most to pull the trigger on “This character isn’t a hero anymore, he’s a villain.”
I haven’t read any Hellblazer/Constantine stuff, but he seems like a good candidate based on what I do know about him. There’s also potentially Etrigan. Like, I guess he’s kinda evil, but then he’s still usually the guy you’re rooting for. Whereas I think others like the Red Hood or Lobo make better antagonists than heroes, Etrigan is just a pitch-black anti-hero contrasted with the more straightforwardly heroic Jason Blood. Very close to the line in any case.