He went crazy.
when did he became crazy ? ... and didn't mana implant the 4th memory into allen , but Earl was the one who killed the 4th ~
It's just a theory, but I'm guessing that maybe the Earl took on both roles since Earl=Mana. The akuma that was supposed to be Mana could be an Akuma that has been programmed to act as if it was Mana while the Earl shows himself in his true form. I think that most probably Mana was losing the battle against his inner Noah, and influenced by the Noah to destroy the 14th, Mana killed Neah. After that he went mad and decided to escape the pain and guilt of killing his twin by erasing his memories of being Mana.
I know I'n getting somewhere way ahead of myself but this is as good a guess as any. Until sensei updates the manga, we're all gonna keep guessing.
Btw, Neah was the one who implanted his memories into Allen as shown in the flashback in the hotel room scene just b4 Kanda barged in. Mana probably met little Allen by coincidence and raised him, but I'm sure someone is orchestrating everything behind the scenes.(〜￣△￣)〜
WTF （￣へ￣）
But why would he forget?