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.

Probably; in a tv show she'd have the time to do both. Then again, Magneto actually does a good job of ruining Bolivar Trask's credit with the government and the sentinel design... and then sabotages himself via football field and not knowing where to stop.