Dan has mentioned multiple times that he had a person whom he 'loved' or had a crush on. As everyone suspected this 'crush' was a guy who went to the same highschool with Dan. Dan became obsessed with him not really in a romantic sense but in a sadistic/possessive sense where he enjoyed watching his 'crush' suffer. Dan was even physically violent with him to the point where he assaulted him multiple times in the past, he developed a taste for it and desired to murder his crush with his own hands. However later his 'crush' died due to mysterious circumstances (Dan did not kill him). ALSOOO when Dan got arrested in the future, he revealed that he did not murder and torture all those 'bad people' for the sake of justice or good deeds. He did it to quench his bloodthirst and his unfulfilled desires, since he was unable to murder his 'crush'.
I just found out about Dan's past and guys he's not what he seems at all. I'm actually soo disappointed and horrified that Dan turned out to be that way since I loved his character and I thought he was really interesting.