Formal:
* Beheaded: This is the most common formal synonym.
* Severed: This is a more neutral term, emphasizing the act of cutting.
* Dismembered: This term emphasizes the removal of a body part.
Informal:
* Chopped off: This is a more casual and graphic term.
* Lost their head: This is a figurative expression, often used in a humorous or ironic context.
Euphemisms:
* Met an untimely end: This is a more polite way of saying that someone was killed.
* Suffered a fatal injury: This is a more clinical way of describing the cause of death.
The best word to use will depend on the context and the tone you are trying to achieve.