Both of the main characters have lost their memory and find out to have a very dark past that they didn't know about. In Psychopath Diary, the main character finds out that he killed multiple people after reading a diary (which wasn't his, but he didn't know that), while in Forgotten the main character finds out that he had killed a family in the past, because the police officers and psychologists who live with him tell him.
Another similarity is that after losing their memories, both characters live in their old house in order to recall the memories that they lost.
Both of these characters end up confused, not knowing who they are, or who they used to be before their memory loss.