"
Well, I shouldn't have to justify what I do.
I shouldn't have to prove anything to you.
I'm sorry that I exist,
I forget what landed me on your black list
Marceline doesn't want to have to justify or prove herself for something she just does on a regular basis; being a vampire. When she sings "I'm sorry that I exist I forgot what landed me on your black list", she's saying that she doesn't think what she did was wrong and that she won't justify it. She's apologizing for being a vampire, and maybe suggesting that P.B doesn't like the fact that she is a vampire, and maybe she's using the sarcastic tone because P.B is cold towards her because, being a vampire, she is literally cold. Marceline doesn't care that she's a vampire, and maybe this is why Marceline and P.B's friendship is tense and very short held. Marceline just wants to be friends, being a vampire, she's lonely.