spoilers:end of spoilers
This isn't inherently a bad move, because that potential is built into the character of Magik. This is something she struggles with literally every day, and her story is about not giving in to the darkness in her soul and becoming the same as her abuser Belasco. On the other hand, everything about how she's been written in modern times has been leading her in the opposite direction, especially Labors of Magik and the recently concluded Realm of X, and would throw out all of the character development of the last few years. It's not like it's the 80s where she was just a kid who couldn't control her power and could be a powder keg from bottling up all her emotions or even the Gillen version from over a decade ago who had out of nowhere given up on even trying to be better. For the last 10 years she's been on an arc of healing, gaining more control, and becoming more heroic and a better person, and has kept herself under control even when her brother and Kitty were killed, so it's hard to see what kind of inciting incident could make her snap for good now. If this is true, they will need to do a lot of work to make it seem like something natural and not a random change made solely because of some silly editorial whim. There needs to be a damn good reason, assuming it's true.
The other issue is it's such a cliche at this point. We seem to just now be ending years of Beast as a villain, after Emma's years as a villain during the IvX period, Cyclops as a villain in AvX, and so on. In the 80s it wasn't such an overused trope and the prospect of them repeating what they did in The Dark Phoenix Saga actually carried some excitement and dread, whereas now it's been tired for many years and even Captain America has had a story like that with the hated Cap was Hydra all along event.