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Once Charles lost his legs, he found the edge that Erik keep asking for. The edge to be willing to twist his powers for his goals. Charles never did it for big things, like terrorist acts. However, small things, like manipulating the family lawyers to sign for the school with no fuse or giving a parent a 'nudge' in the right direction.
Charles began to cross the line. Its never good to cross a line when you're a telepathic. It leads to terrible, terrible temptation and Charles was never one to resit temptation. Erik got what he wanted in the end. Charles embraced his powers. But he would never be Charles again.