This role is third-party by definition.
A popular variant of Arsonist is more like a modified Vigilante. This Arsonist's Night Action is to prime a target player. Its secondary action is to ignite all primed players, killing them all at once. Ignition may occur during the Day if the moderator allows it. There is no consensus as to whether the Firefighter protects players from being primed. One-Shot Arsonists are only allowed to ignite once. This variant is more likely to be Town-aligned.
Use and Power
The Serial Killer version of Arsonist is midway in power between an ordinary SK and an Unstoppable SK, thanks to the Firefighter being the only role that can protect against its kills. Other than that, it's mostly used where flavor calls for it.
The variant is a weaker but less swingy version of the Vigilante in that it can choose not to ignite its victims after priming them (or it can die before it gets the chance to ignite them).