Vengeful Townie

From SA Mafia Wiki

A Vengeful Townie is a town role who, if the victim of a lynch, can perform an immediate daykill on any other player before the cardflip. If the lynch is prevented by another role, such as the Governor, then typically the vengeful ability does not trigger.

Oftentimes the kill is restricted to only the players voting for the townie.

If the player automatically kills whomever was the hammer, then that's called a Lynchbomb.