A pyromaniac is a person with pyromania: they feel a powerful, sick need to start fires. It sounds like a doctor's word, but crime reports and films use it too, sometimes too loosely, for anyone who sets a fire. Starting a fire to get insurance money is a crime with a plan. A pyromaniac, in careful talk, is driven by the urge itself. Use this word about people, not about the illness. Though actually, calling a child with a lighter a pyromaniac is unkind and usually untrue; the real term is meant to be heavy.