I would add surprise and shock to what Ryl said. As Ryl pointed out, Itou NEVER expected Sekai to do that to him. Bastard deserved it.

I wonder what happened post-series. Its obvious Sekai's body was to be found during a class day. Would the other girls try to seek Itou, to tell him, and find his headless body? Will people blame the crazy Katsura for both deaths despite only causing one? Would she be sent to prision? So many possibilities left to the imagination.