Its not about being good..the x men went his direction for years..Cyclops was a fake Magneto 2.0 with Jean off the chess board. He was a X man in 09 because he liked Cyclops' direction and choices and didn't feel like kicking his ass over it again.
Mystique and Sabretooth are never part of the big picture so they don't matter.
Villain terrority? Sure. The Magneto series from Bunn...he killed all of those anti mutant humans and Shield couldn't do shyt about it..lol. He killed Red Skull even Wanda pleaded with him not to do it. He always goes the extra step.
Rogue and Magneto made sense considering she was good as dead without Magneto.
even with scott taking on some of his traits, he was also taking in charles. going full magneto would have lost him everyone but emma. and he was an xmen because scott was taking in most mutants at the time.
mystique is currently a massive part of the big picture. in fact shyt can come crumbling down if she decides to take action.