Someone who loves to set fires — and, for whatever reason, can't stop setting them — is a pyromaniac.
Maniac is a word attached to many words for mental illness, but this has to be one of the most scary. A pyromaniac has a frequent, powerful desire to set fires. This is different from an arsonist, who sets fires for money. Pyromaniacs just set fires because they want to and feel a compulsion to. Pyromania is a sickness. A pyromaniac could also be someone who loves watching fires.
