@Shinta: You're right, I had forgotten that Itou recognized and admitted his own wrongness in one of the episodes around the School Festival. With that there's no more doubt that he did possess a normal moral compass at some point and chose to ignore it. I still have to blame Sekai some for facilitating his original cheating, but by the time of the School Festivel he knew that leading Katsura along while cheating with Sekai was not something he should be doing but he did it anyway because it was easier. That's pretty despicable (or pimp depending on how you look at it).
Yet, even with his horrible treatment of other peoples feelings, no high school aged guy deserves to die for shirking social responsibilities. Nothing Itou did would have caused harm if he didn't have the implicit cooperation/trust/support of his victim. He deserved to face consequences for his betrayal of that trust. However, he couldn't hurt anyone who didn't choose to trust him (Otome and Hikari turned out just fine), thus he was not any kind of threat to society at large so I see no justification for his death, other than Sekai's satisfaction that he suffered for what he did to her.