There are generally two types of X-Men villain. On the one hand, you have characters like Magneto and Emma Frost, whose immorality is undercut by a spark of goodness and a code of ethics which, a lot of the time, sees them working as part of the X-Men rather than fighting against them.
Then there are the villains on the other hand, the real dyed-in-the-wool-but-specifically-dyed-red-with-blood bad guys. These characters aren't doing what they think is right, they're people who turned to crime to indulge in their twisted obsessions, exercise their raging prejudice or to just do some good old fashioned despotic world conquering.
All of them have more than earned their reputation as monsters and have committed some appalling crimes to earn a spot on this list. There are men who want to wipe out the entire mutant race just for existing and mutants so deadly you’d be forgiven for agreeing with those men.
Either way, no-one on this list should be expecting an invite to the next X-Men baseball game.
It's pretty easy to sum up why Azazel should be on this list, he's literally the Devil.
To elaborate, Azazel is the leader of an ancient race of demonic mutants called the Neyaphem who were banished to an alternate dimension by an angelic race of mutants called the Cheyarafim. This story apparently formed the basis of humanity's ideas about Heaven, Hell and Satan (well, except for the fact that Heaven, Hell and about a hundred different Devils already exist in the Marvel Universe).
Anyway, thanks to his teleportation powers, Azazel was able to briefly return to Earth periodically. However, the only way for him to permanently return and take over the world was with the help of his children who are "linked to his dimension" and so he used his Earthly visits to do the nasty with any woman he could seduce. It's a plan that's as creepy and sinister as it is completely nonsensical.
As some of you may have guessed, one of his children is the X-Man Nightcrawler. Now, obviously, the preying on innocent women and the attempted world domination are both pretty bad, but Azazel's real unforgivable crime is linking fan-favourite (1/1 article writers agree) Kurt Wagner to this god-awful idiotic plotline.
More recent stories have wisely chosen to ignore a lot of this and instead portray Azazel as a traditional demon. Of course, that may make you wonder what makes the rest of this list so evil that the Devil himself is at the bottom. Well, read on.