Allen got the crap end of the deal yet again
Basically, Allen discovered after the Re_Birthday that, for whatever reason, he'd never truly be permitted to be with Riliane again, with her being reincarnated as Rin in the Fourth Period and him having to stay with Levia and Behemo and the other gods. As the weight of everything he's been through finally collapsed upon him, he completely and utterly snapped, furious that even after all of that he can never have the one thing he truly wanted—to finally, finally be with his beloved sister again—Allen decided that the world was just irredeemably unfair and sought to make everyone suffer as he suffered.
Alternatively, this "Allen" isn't really Allen
Evillious had plenty of people who used the identities, names and even faces of previously known characters. Why should it be any different here?