I agree with pretty much everything you've said.
We went into Mansions of Madness with the spoken (and oft reiterated) understanding that "This Game Is NOT About Combat". The players knew that going in and were fine; a nice change of pace.
However, MoM then betrayed their Covenant of Pacifism by demanding they kill a named maniac with a buttload of health; a game not designed to focus on combat suddenly forced them to focus on combat. It's hard to begrudge a little peevishness.

