Comic book characters tend to fall into either one of three brackets as do-gooders, evil villains, or filling some role in a morally ambiguous in between. There tends to be a lot of overlap, and when there is, it's often seriously entertaining. Villains becoming heroes. Anti-heroes becoming anti-villains. That sort of thing.
But possibly the most interesting turn is when heroes become villains. It's a rare sight to behold in Marvel and DC's respective main continuities, which has led to the trend of Elseworlds and various What If...? stories broaching the concept of what would happen if the most kind-hearted protagonists suddenly switched moral allegiances. Factor in the frankly absurd power levels of some of these figures, and it's probably even fair to say that they end up being twice as terrifying as the characters who take up evil as a full time job.
Sure, this specific trope can start to feel a little stale when it's the same heroes doing it over and over again (I think everyone's suffered enough evil Supermen for one lifetime), but every once in a while a creative team arrives to make it all feel worthwhile - especially if said change is long lasting.
From brainwashed apocalyptic horsemen to genocidal doppelgängers, here are the heroes that proved that being bad can also be so, so good - at least for the reader.