IPA: /ˈɑːrsənɪst/
KK: /ˈɑr.sən.ɪst/
A person who deliberately sets fire to property, usually to cause damage or harm.
The police arrested the arsonist after the fire destroyed several buildings.
Arsonist → It is formed from "arson" (from Old French *arsion*, meaning the act of setting fire) and "-ist" (meaning a person who performs a specific action). An arsonist is a person who commits the act of setting fire to property intentionally.
Think of someone who performs the act of setting fire ('arson') — the '-ist' indicates that this person is actively involved in that action.