Reposted from my comment on this meta on Erik and repercussions:
A few people mentioned that Erik’s supervillainy began the moment he tried to kill Mystique, that, while in character, it was a ridiculous move that marked him as a terrorist and a monster, and I’d like to disagree.
At first blush, I somewhat agree.
However, Magneto, skilled strategist, should really be thinking beyond the first blush.
Is Erik going to track down and kill every shapeshifter alive, and every shapeshifter who manifests their mutant powers from then on, to keep them out of Trask & Co.’s hands?
And as Helens78 pointed out to me, it’s not just shapeshifters. What about Rogue? If it’s dangerous for Sentinels to have Mystique’s ability to adapt to mutant powers, imagine how dangerous Sentinels would be if they could absorb and use mutant powers directly.
And once you decide some mutants are acceptable sacrifices to preserve the rest, why stop there? Telepaths are obviously dangerous— Magneto already uses a helmet to protect himself from them. Their powers could be turned against other mutants, and if Magneto talks to Wolverine about the future enough to hear the plot of X2, he’ll learn that Charles’s powers will be used that way. So time to kill Charles, too, and then every other telepath. If Magneto keeps asking, he’ll learn that Wolverine himself has been turned against his fellow mutants, so here comes that rebar-and-river plan, for the good of mutantkind. And so on. Though I imagine that curiously, this pre-emptive strategy would never apply to Magneto himself, who of course could never be turned against his brother and sister mutants. The ones that survive.