Martyr

person who suffers persecution and death for advocating, refusing to renounce, and/or refusing to advocate a belief or cause, usually a religious one

A martyr is originally a witness and now means a person who died or was killed because of their faith, for example Saint Stephen. It is often used for people who died for any belief or idea, especially in politics.

The Christian Martyrs' Last Prayer, by Jean-Léon Gérôme (1883).