This plot with Grotesk is largely an excuse for something far more interesting to happen. Professor Xavier has become increasingly odd. He's being very harsh on the X-Men in training and is conspiring with Jean about something. Scott even has started to wonder whether Xavier, like him, is in love with Jean, but let's not think about that. His telepathy is possibly wonky (he's able to abstract out information about Hank and Bobby's initial encounter with Grotesk, but has ceased to mass broadcast commands like he used to).
In the end, he goes alone to stop Grotesk, gets quite badly injured, and reveals his secret. He was dying, of some incurable illness. And then he dies.
It's easy to see why they've killed him. Xavier is rather overpowered compared to the rest of the team. All too often he has acted as a 'Professor X Machina' (sorry), coming in at the end of a story and fixing everything. His recent kidnap allowed several stories that would have been impossible otherwise, and after one story with him back, they've swiftly made the decision to get rid of him for good.
But somehow, I don't think this is going to stick.