I don't know why, but characters like Bennett always get to me more than characters like Sirius. Sirius is most likely either the worst person in Reborn or second-worst after Lin, but it's not the merciless house-burner and soul stealer that sets me off. It's the misguided kid who's easily influenced and has a poor control over (and possibly understanding of) his own emotions.
I've noticed this in a lot of games, books, etc. The people I absolutely can't stand usually aren't the major villains that are global-level threats.
They're the random throwaway assholes whose mouths and egos are bigger than their threat level, the school bullies, the abusive parents/guardians, the confused/misguided ones who are easily influenced, etc.
With someone like Sirius, it's like "You did bad thing or are trying to do bad thing, and I'm going to stop you!" and that's the end of the emotional investment. He needs to be stopped, but his actions, goals, and words don't make me angry on a personal level. But with Bennett, I want to rip him to pieces.
The best explanation I can think of for this is that it has to do with the way the characters carry themselves. Usually, those categories of villains I mentioned not being able to stand just act petty and disgusting and ridiculous (not in the good/fun way) and have very little if anything for me to like about them, while the main villains have a better chance of acting like a boss and/or having a personality or history that catches my interest.
In Bennett's case, he also kind of reminds me of creeps who can't take no for an answer which is why I said if I were Luna I'd slice through with him with Bisharp. I'd feel threatened enough to do it by reflex, and anger enough to do it even not by reflex because I despise those people.
I do think Bennett will be redeemed, and I'm looking forward to when/if it happens.