Here's why:
* Verb: The base form is "behead," meaning to cut off someone's head.
* Past Participle: The past participle is used to form perfect tenses (e.g., "He has been beheaded") and passive voice ("He was beheaded").
While "beheaded" can sometimes be used as an adjective (e.g., "the beheaded king"), its primary function is as a past participle.