If he actually got installed by the House, there'd be widespread unrest. We know he knows that's not a realistic option, because a) reps would never do something that would upset their constituents so seriously and b) if they did, there's a real likelihood of armed resistance. So what's the end game here? He just felt so bad that Utah may go Trump he felt he had no option but to step in and make sure that it didn't go down in history as a pro-Trump state? That's the only benefit McMullin can offer, and it's only valuable if Trump gets elected and actually turns out to be Hitler 2.0. Then Utah can have a clean nose.
In practice the only thing McMullin is doing is making a Clinton presidency more likely. Ross Perot served the same function in 1992, taking ~20% of the popular vote, probably 15% of which would've gone to Bush had Perot not been in the race.
Given McMullin's background as both a CIA operative and an international banker, I think it's fair to ask just where his intentions lie.