You happen to know, experimentally, that beings with a high EQ tend to commit actions that decrease general utility in the population whose utility you care about.

You know a lot more than that. You know that they go to different afterlives than Good or Neutral beings, that they can be affected by different spells and abilities, and that depending on their class their own abilities might be affected by their evilness.

A moral theory that supports the eradication of Evil beings need not be utilitarian. I don't think a conventional paladin would function as a utilitarian, for example.