Both the term "arsonist" and "pyromaniac" refer to acts of crime. An arsonist is someone that sets the property of someone else on fire, all in the name of revenge. It is a real criminal offense with a mindset of vengeance. The real crime from which arsonist is gotten from is called "arson." Arson is defined as an act of setting someone's building or property on fire due to preexisting malice.
This is responsible for a massive loss of property with high monetary value in several countries. Most arsonists are victims of bad parenting or any similar family problems. On the other hand, a pyromaniac is someone who commits a crime due to several rates of impulse. They are being disturbed by a wave of impulse, which causes them to act in such a disorder manner.
A pyromaniac is usually found doing something uneven, often with excitement. They are usually seen to be acting out of tension and excitement while doing something uneven, and later feeling relief afterward.