Disclaimer: I don't own any part of the X-Men franchise (except for some first sale rights to some excellent comic books.)
A/N: This was written in response to the prompt:
X-Men First Class, author's choice, mercy died on a desolate stretch of beach when one of Moira's deflected bullets killed Charles Xavier
The Next Extinction Event
The balance between rage and serenity, Erik thought, shouldn't feel like this. Rage, he knew, though it had been a very long time since it had reached this peak. But this was not the serenity that Charles' had shown him.
Charles had helped him remember a time when all was well with his world, when he had been warm and happy and loved and nothing needed changing.
This time, collapsed on a beach in Cuba, cradling Charles' body in his arms, he felt serenity of a different type. It was cold and empty with the knowledge that nothing he could do could possibly fix this.
This cold despair was a still point around which the rage circled like a growing storm. The longer he sat there, the stronger it got. The still point more colder and more leaden, the rage faster and hotter.
...
It wasn't until much later that Erik realized what he had done.
After pure physical exhaustion had finally knocked him out.
After he'd managed to get to civilization and read the newspapers.
That was when he realized that his power wasn't to control metal, as he had thought previously. His power was to control magnetic fields. A suitably strong magnetic field stops hearts, a lesser one stops electricity, and a more pervasive one shifts the rotation of the very planet.
Charles would not have wanted an ice age as his funeral pyre, but Magento thought it only suitable. Having killed such a great light as Charles, those who survived must do so in darkness.