Behead, beheaded, beheading

Why behead? Why not dehead, or unhead?

Other words with the prefix be-: bejewel-put the jewel "on"; bespectacle-with the spetacle "on"; befriend-to become friend, put "on" friendliness; beguile-to put "on" guile; befool, belittle, bewitch, bespeak, and so on.

Notice the emphasis of on, against, around, over, to, that the be- prefix adds to the stem word.

None of them uses be- as "off" or "away" or "without."

Perhaps the verb "to behead" was originally applied to the beheader, who is adding another head "onto" his collection. No?