i think he shoulders enough blame for his actions that he's beyond any sort of forgiveness, personally!! but it's not like i'm saying shido doesn't shoulder blame, either!! they're both equally guilty, if you ask me!!
though, i do agree that the way he was handled was pretty bad!! it was super unsatisfying to get all of his characterization at once, and then to just have him just die!! on a personal level, i also really hate that it was a heroic death!! either way, it would have been way more satisfying for him to continue on and attempt to atone(and, preferably, suffer a lot in the process!!), or at least have the sacrifice scene occur later on!!