A death note would be so broken... I wouldn't put it past the gods.
I don't doubt that there's a town role that gets a benefit from correctly killing a mafia. That actually seems very likely to me. Some of us have discussed this informally in the past, but there's only so many ways you can reasonably change things up given our current framework of roles/items and the types of things that they do. E.g., past roles and items are some linear combination of investigation-like abilities (investigator, superhero, genie, microphone, gay knight), invincibility-granting (medkit, conspiracy theorist, cynic, superhero, taser), innocence-establishing (gay knight, innocent child), role-information (seer, genie, vampire, police officer), mafia-guessing (conspiracy theorist, vigilante, taser), vote-manipulating (fake ID, mayor), etc.