In that scenario, the utility value for the utility monster would have to be enormous. We're talking something that would probably also result in human deaths if it were to go unhandled, like triggering a war or nuclear meltdown.

I feel the greater ethical flaw in utilitarianism is the case where a utility value scales positively with suffering. What can you do if the utility monster derives positive utility value from others dying, as opposed to remaining linear? Utilitarian measures simply don't fare well when the actors are inherently antagonistic to one another.