Is every cop that's ever arrested a criminal, or retired from crime fighting, a potential mass murderer?
Especially those cops that aren't being paid and sworn to the line of duty, and are only volunteering to put on a spandex outfit because they can lift a semi.
Realistically, if you remove the inevitability of a villain's escape, this question is ridiculous, because applying real world logic (which is the mindset our heroes are supposed to adopt), there's no reason to assume their escape.
And, if you're going to say kill them because they're obviously just going to escape.... if they're popular enough to be written out of jail, they'll easily be popular enough to be written out of death. Making the superheroes nigh useless.
We can't judge Superman because comics need recurring villains.