In a few words, this is the dark version of Angel Beats. What I could understand watching this is that no matter what happenned in your past, it's still a part of you that you can't just get rid of it. The message of the anime is: "You can't just get rid of your past. To continue progressing in life, accept your past and get over it. Otherwise, you will stop in time and live a life without being really you. Your past (traumas, regrets, pendences...) is a part of who you are".
