Knowing Magneto's way of thinking, he probably thought Jean and the others might not be able to do whatever was necessary if Jimmy could not be saved.
Even Jimmy surviving the X-men will be angry with Magneto for him acted on his back the same way that always happens with anyone who works with Magneto. That's why I like Magneto he does not hide what he is and makes clear what can be expected of him so when the inevitable betrayal happens no one can say did not expect it. It is the fault of the good guys for always ignoring the truth about him.