An arsonist is someone who likes to put things on fire. Not just like but sometimes obsessed by it, like an addiction. A serial arsonist is what the firefighters and police talk about when they have to deal with someone who keeps setting houses on fire. It can be very dangerous and eventually usually someone dies. For the firefighters this is somewhat a nightmare since they have a problem dealing with the arsonist and at the same time have the normal alarms coming in at the same time. It's crucial they find him before someone dies.