Alright, so I only read a few episodes. From my understanding, he just straight up murdered a couple people, then taunted the girl who said he was a monster for doing so (she ain't entirely wrong) with the exact body language a violent psychopath might do with a victim.
I think by definition, maybe he's an antihero? He's the protagonist of his own story. This story is about him from his perspective. If the story itself takes the view that he's fighting against a world that is against him, then I think it makes anyone standing in the way of him and his goal the villain. I'm not crystal clear on actual definitions of antihero vs villain, though. It seems like a lot of different people who write about it see different things.
Personally... I'm someone who views life and death, and murder, a little different from how the generic No Kill superheroes see it, and I do think there's quite a few things that deserve harsher punishments. I have some extreme views here and there when it comes to certain crime. But Dan is 100% completely in the wrong.
Is bullying cruel? Yes. Can it damage people long-term? Yes. But should you be literally murdered for saying some unnecessary, harsh things to someone? Absolutely not, and that's someone who has experienced bullying and other forms of violence. These are not kids who went around, torturing Dan from birth, or attempting to murder him by beating him for hours. There is no government-sanctioned and socially-accepted, system-wide attempt to torture this one boy.
The kids said some stupid, unkind things, and while they should unarguably face reprimanding for it, what they received in return is a thousand times harsher than what they deserved.
Dan's response is reminiscent of those shooters who go to school and kill people because they think everyone is against them. His actions are heinous, and indicates he is extremely unstable. It's not like he killed someone, then reacted in shock and disgust at the action, it was more like, "WHOA, cool, I can do that!?"
I would also be very concerned about those who think he was in the right.