Magneto was probably worried about them, too. But you don't need to warn Storm about the dangers she's already aware of. He wanted Storm to keep her...
I mean, can you betray someone who's already betrayed you? I feel like the 'being betrayed' part removes any obligation to stick by your former ally...
More like they've given up on the way Xavier did things (they have no choice really considering his recent antics, Peaceful Co-existence seems to be...