If I give my friendship to someone, I’ve befriended them.
If I give my love to someone, they are beloved.
If I take away someones lice, they are deloused.
so why is it when ones head is chopped off, it’s called beheaded?
Compared to the other uses of the prefix, it sounds more like they were given head (no pun intended!:D)
Why isn’t the correct term deheaded?
I’m still trying to figure out why you can’t be capitated, combobulated or fenestrated.
If you are benighted, you are inconvenienced by night-fall.
Maybe if you are beheaded, you are inconvenienced by head fall.
From the dictionary.com listing for “behead”:
beheaded
adj : having had the head cut off; “the beheaded prisoners”
Describe?
I’ve wondered about this denotation as well, but the connotation of DEheaded seems somewhat clinical or surgical, sort of like deboning a fish.
The operative usage of the prefix be- in this case relates to the privation of the head. From the OED entry for be-:
Words such as befriend and beloved are using the more common usage of the prefix, and are much more . . . er, common. 
Beheaded = Beware: head begone; bereft of head; head befell in a bedpan!
Becalm.