For what it's worth, I agree with Ultron.
If I was HHA and I knew I could sacrifice an HHA to get rid of an Investigator, I would do so, even if it puts me at a drastic numerical disadvantage. Even better, if I could get an eviction going in the day, I would build a huge amount of credibility for myself by taking out an HHA immediately. This is a strategy known as "busing" (as in, throwing under the bus) in the more hardcore Mafia circles.
However, I am a bit of a power gamer, like I feel ultron is as well, and my viewpoint might not be shared by my hypothetical HHA compatriots. Everyone is well aware of the opposition I received to evicting the roomies on Day 1, and the HHA need to reach a consensus to evict (or at least a pluraity, however it works in this game) as much as we do.
And, of course, it's in ultron's and francompagne's interests to stay in this game. I still believe that they have a win condition that triggers when they're the only ones left. So, if ultron was a HHA in a one-man house, he probably wouldn't hesitate to argue to evict another HHA just to get rid of the investigator, but the dynamic changes when the power gamer himself is at risk of being sacrificed for the "greater good".