I don't think what Akiba did was significantly worse than some of the other characters, but he's the only one who doesn't get a happy ending. For example, Adachi seems to torture people just for fun, and sexually assaults Shibuya throughout the entire series, even after she becomes an ally. She's given a tragic backstory, and then all is forgiven.
For Akiba, the main cast decide to just cut ties with him in the hopes that he'll reform, but he's a psychopath, in the literal sense that he is incapable of understanding other people's feelings. There's no way he can get better without outside help. Shibuya realizes this, though she can't be blamed for not trying to help him after what he did to her. In the end, he's stabbed to death, understanding nothing.
Maybe that's the message the author was going for, that some people are just born with it and can't be redeemed, and they're different from people who are just messed up from a tragic past. Still, it leaves a bad taste in my mouth. If he was just evil and refused to apologize that would be one thing, but I wish Akiba could have at least been able to recognize that what he did was wrong by the end. I don't like that he gets killed for something he can't control, despite everything he did. It's just sad.