the zodiac killer would write a lot about how killing people made them his slaves in "paradice", but i believe he wasn't sincere about it. i think he was just sensationalising his own murders to bring more attention to him. he lied about being the culprit behind a few crimes at least, so he is obviously an unreliable narrator. he knew that saying things like that would increase the fear and mystery around him, and it never truly seemed like he meant it in any of his letters, even if he might have wanted us to believe he did.