She really had an urgent need to do something.

Which I think really is a big weakness for a player whose plan has complicated consequences like Mystique. Charles doesn't make his case well when he tries to talk her out of her plan, but generally vigilante assassination rarely works out in history. I like Mystique being badass (though I hate the choreography of her fighting), but I wish she, in the role of an international spy, had considered a more, uhm, subtle course of action, regardless of her relationship with Charles.

(But from a Doylist perspective, it's probably the logic of blockbuster movie that there demands opposition = killing, instead of something less flashy like Mystique sabotaging the Sentinel design and/or Bolivar Trask's reputation.)