Charles was perfect.
He was rich, popular, and kind. Everyone who met him loved him, and those who didn't know him have heard great things about him. No one had a bad word to say about him.
Maybe that's why Erik loved it when someone started an awful rumor about Charles. Poor Charles got his name besmirched- something about breaking some poor girl's heart. She had probably been some one-night-stand who couldn't accept that Charles didn't fall madly in love with her.
Erik wanted to ruin him.
Erik wanted to see Charles's face become a ruined mess and his good reputation shattered under the weight of the public's judgement. He wanted this, as if somehow Charles's worth was tied to his beauty and public persona alone, so that no one would want him. His adoring fans would abandon him in his time of need, and Erik could save the day- never mind that he had watched Charles's downfall with glee, lying in wait. Charles would be none the wiser.
Yes, Charles with molten bruises covering his once-flawless face and a reputation for breaking girls' hearts would be wholly unappealing to anyone but Erik.
Erik would finally have Charles all to himself. The telepath would have no choice but to love and appreciate the metal-manipulator, and Erik would get everything he's ever wanted.
He'd get Charles.