Marvel made their mark on the comic book industry by creating complex characters with lots of depth and storylines adults would enjoy reading nearly as much as the youngsters.
Some of Marvel's characters are so complex it's hard to decide whether they're the good guys or the bad, or an anti-hero purposely bridging the gap between the two sides.
Take Professor X for instance- he's the founder and leader of the X-Men who is responsible for protecting mutants from mankind (and vice versa) so we naturally think of him as a good guy.
And yet we discover the Prof has a dark side when he tries to psychically murder Magneto and instead combines with him to form the super-killer Onslaught.
Plus, during the Deadly Genesis series, we find out Xavier "used the abduction of the original X-Men by the living island Krakoa as an instrument to discard the first-generation X-Men of 1963".
On the other hand, it's hard to tell if Nick Fury is a hero or villain because he walks a thin line between good and bad and never hesitates to put a bullet in anyone, hero or villain, who stands in his way.
But who could have known he would kill an innocent extraterrestrial being just to catch a villain?
Nick Fury's murder of the Watcher Uatu in the Original Sin series proved he is truly Marvel's most morally ambiguous character, a guy who's not afraid to get dark if it means getting his way.
See 5 Beloved Marvel Heroes With Seriously Villainous Pasts here