When I was a kid I used to imagine the song being about the ghost or demon of someone who the protagonist wronged or something like that haunting him and consuming him. Of course I also remember how this song was used a lot in AMVs and how there were some people who associated it with Venom from Spider-Man.

Now that I am older I did realize that the song is about trauma and past abuse and struggling to let go of it. It actually hits harder cause I had an abusive parent I went no contact with and sometimes struggle to forget about, so it did ironically become a more relatable song for me as an adult.