Most good guys who are part of the X-verse who turned bad, only did it for a short time and due to other factors, being influenced, possessed or forced into attacking the X-Men. There was done a voting thread about those, of which the results are revealed this week.
But there are also those who choose to change. Like WWII heroes Stonewall, Commando and Super Sabre, who volunteered becoming part of Freedom Force.
Or former X-Men Vulcan, or Bishop, who became enemies of their former teammates.
There are not many. So please submit a top 10 of favorites, or a top 5 if you really can not come up with 10.
These are the ones I could think of. If you have others, just add them to your top 10 and I'll add them to the list.
Amelia Voght
AoA Husk
AoA Iceman
AoA Nightcrawler
Bishop
Colossus as Acolyte
Crimson Commando
Esmé Cuckoo
Garrison Kane
Grizzly
Joseph
Lionheart
Madelyne Pryor (as Red Queen, THAT was her own choosing)
Mikhail Rasputin
Mortis
Rory Campbell becomes Ahab
Rusty Collins (Firefist), joining Acolytes
Skids, joining Acolytes
Stonewall
Strongguy, becoming Supreme Hell Lord
Super Sabre
Tier Sinclair
Ultimate Jean Grey
Ultimate Psylocke
Vulcan
Wallflower in House of M
Wither
Zero (Kenji)