The fact that Miller was traded and not Pettersson doesn’t mean that Pettersson wasn’t an issue in his own way, and Allvin’s own comments calling him out in December confirm this.
You can easily just look at the age and likely improvement of Pettersson and likely decline of Miller to justify the trading of Pettersson over Miller.
Personally, I don’t think one needs to be “worse” than the other. They each had/have issues. By the sound of it, Miller is uber competitive and can be hard on his teammates as a result, and seems to be unable to restrain his own emotions. Pettersson seems to have effort/compete/preparation issues as confirmed by his own GM. Both of those are problematic and it’s actually not hard to see why, in addition to personality differences, they didn’t get along.
I think everything actually points to management and coaching actually preferring Miller’s will to win and attitude in that regard, but I also think it’s clear that Millet just couldn’t leave Petey alone despite Tochett telling him to worry about himself. And I think that was management and coaching’s biggest issue - Miller repeatedly ignoring them.