Just because Erik was angry.
One of my many headcanons about the beach is that even before Charles says No, my friend, we do not, Erik already feels betrayed and resentful.
Erik has lost so much in his life, everyone he cared about. He fought hard against feeling anything for Charles. Erik only let himself have him after he witnessed just how powerful Charles is. Telepathy, memory manipulation, mind control… once Erik got firsthand experience of all that power, somewhere deep in his lizard brain, he became convinced that Charles was so strong that no one could ever hurt him. Consciously he knew better, but emotionally, he needed badly to believe that it was safe to care about Charles, and once the assumption rooted in his mind, he let himself feel everything for him.
And then Charles failed him by being only flesh and blood after all.
LET ME JUST DIE RIGHT HERE, THAT’S OKAY.