Superhero comics can be identified through an array of pretty obvious signs: bright costumes, secret identities, ominous narration - but one element that will always be present is the dichotomy between hero and villain; those that operate on the side of good, with or without powers, and those who work on the opposite, sewing chaos and destruction wherever they go.
They're a constant in the genre, but there are times where supervillains have briefly dabbled in superheroics, divesting from the dark arts and joining forces with their nemeses in search of a common goal. Some, in rare occasions, even make the switch permanent, so much so that their time as a supervillain has actually been forgotten by the general public.
It's not as though these switches have been limited to one publisher solely either. Both DC and Marvel have exercised this trope in the past to varying degrees of success, but when done properly, it's hard to think of a better one present in the superhero genre.
So, with double-crosses, changes of heart and romantic entanglements aplenty, here are the times supervillains proved that being a hero is worth the trouble - briefly, or in the long-run.