Thinking about what the ghost of Ichihashi said to Tobio:
"You traitor."
I don't think this refers to Tobio going out with Renka after Ichihashi confessed to her, or to the fact that Tobio was responsible for the bombing, but rather refers to the fact that Tobio is happy, or at the very least trying to be so. The fact that Tobio is trying to be happy while Ichihashi and all of the other victims are dead - and many others permanently crippled - means that Tobio is "betraying" them and their memories. It's almost "treason" for Tobio to want to be happy after he's robbed so many other people of their happiness.
I think that this also ties in to what Tobio said to Paisen in Chapter 86:
"You're a murderer! Why should you have such a happy dream!?"
Tobio claims to have been "saved" by Paisen, but that's only partially true. Paisen's words have allowed Tobio to keep living, but they haven't allowed Tobio to cleanse himself of his guilt. Tobio, more than Maru or Isami, wants to cleanse himself of his guilt. After all, he was the one that smiled and felt joy at seeing the Yaba High students die in the explosion that he was responsible for (remember additionally that he was the one that suggested killing the Yaba High students back at the beginning). Paisen was allowed to cleanse himself of his own guilt by spending ten years in prison, but Tobio was robbed of that chance when the bombing was covered up and he wasn't allowed to properly repent.
As another commentor noted, I would say that the ultimate theme of this manga is "The things that guilt does to us, and the ways that we try to manage guilt and its effects." Again, as another commentor noted, this ending is not supposed to be a happy ending for Tobio. Tobio will never have a happy ending, because he will always be burdened by and live with his feelings of guilt - again, especially because he was never allowed to have the redemption that he sought - and will probably kill himself at some point in his life, because that's what his guilt does to him. If Tobio had managed to achieve a happy ending, then the theme that the manga was going for would have been lost. If Tobio had managed to rid himself of the feelings and burdens of guilt, then everything that had happened in the manga would have been pointless.
Tobio will always be haunted by guilt. He will never have a happy ending. That's the point.
That being said, yes, the ending did feel a tad bit rushed. Still, letter grade B+/A- or so.
EDIT: I also want to take a moment to comment on who shares the most "responsibility" for the bombing. Obviously, all four of them share some amount of responsibility, but Maru and Isami arguably share the least amount of responsibility - neither of them suggested killing the Yaba High students, neither of them made any bombs, and neither of them set the bomb that blew the gas tanks - and this is why they feel the least guilty and are able to lead the most normal lives.
Paisen shares a bit more responsibility since he's the one that made the bombs and is the one that set the bomb that blew the gas tanks, but his ten years in prison allowed him to repent for his share of the responsibility. An important thing to note about Paisen's level of responsibility, though, is that he didn't know that he had set the bomb that would blow the gas tanks, and hadn't even seen the gas tanks at all; he was completely ignorant of what was going to happen once they set the bombs off. Obviously he's still responsible to some degree since he was still the one that planted the bomb, but Paisen did not know what would happen. Unlike Tobio.
Tobio absolutely holds the lion's share of the responsibility for the bombing. Not only is he the one that suggested killing the Yaba High students, but Tobio saw that Paisen had set a bomb that would blow the gas tanks - where-as Paisen himself did not - but said nothing about it. He claimed that he didn't say anything about the gas tanks at the time they were setting the bombs because they needed to escape from the security guard, but the last page of the manga tells us the real reason that he didn't say anything:
He wanted the Yaba High students to die. When he saw the Yaba High students burning alive, Tobio was happy. Tobio knew what would happen, allowed it to happen, and felt happy that it had happened. As another commentor noted, his happiness was possible because he viewed the Yaba High students as some sort of foreign "other" that he didn't need to feel empathy for; after all, they had harassed him and his friends and nearly killed Maru, so they were just bad people getting their just deserts, right? It was only after Tobio met Ichihashi in the hospital that Tobio began to really understand the full gravity and weight of his crimes - that is to say, that he had killed other real, living people, with families and friends and dreams and loves and so forth, just like him and his friends - which finally allowed guilt to really begin to set in fully and properly.
Even if he wasn't the one to set the critical bomb, Tobio shares the most responsibility for the crime, and this is why he feels the most guilty out of all of the four main characters. And, as I said previously, poor guy didn't even get to repent properly, so he'll be left with that heavy burden of guilt for the rest of his life, without any way to alleviate it.