Here’s the OED entry with the etymology cut and pasted:
behead, v.
Pronunciation:
behead#_gb_1.mp3 /bɪˈhɛd/
Forms: OE behéafdi-an, ME behæfdien, ME bihaued-en, ME biheafdin, bihafdi, ME bihefden, ME… (Show More)
Frequency (in current use):
Etymology: Old English behéafdi-an , < be- prefix 3 (with privative force) + héafod head n.1… (Show More)
Thesaurus »
1. trans. To deprive (a man or animal) of the head, to decapitate; to kill by cutting off the head.
c1000 West Saxon Gospels: Matt. (Corpus Cambr.) xiv. 10 He asende þa and beheafdode Iohannem.
c1160 Hatton G. Matt. xiv. 10 behæfdede.
a1225 Juliana 40 To bihefden [v.r. beheafdin] pawel.
c1275 (▸?a1200) Laȝamon Brut (Calig.) (1978) l. 13124 Þat heo us wulle bihafdi.
1382 Bible (Wycliffite, E.V.) Matt. xiv. 10 He sente, and bihedide [v.r. byheuedede] Joon in the prisoun.
a1450 (▸c1410) H. Lovelich Hist. Holy Grail xlvii. l. 155 Beheveded On Aftyr Anothir.
1474 Caxton tr. Game & Playe of Chesse (1883) ii. iv. 48 Other sayd that they shold be beheedid.
a1535 T. More Hist. Richard III in Wks. (1557) 54/1 To bee byhedded at Pountfreit.
a1616 Shakespeare Henry VI, Pt. 2 (1623) iv. vii. 93 Take him away and behead him.
1781 Gibbon Decline & Fall II. xlvi. 719 A great number of the captives were beheaded.
1873 H. Spencer Study Sociol. vii. 156 We beheaded 2000 fellahs, throwing their headless corpses into the Nile.
fig.
1593 R. Hooker Of Lawes Eccl. Politie iv. xiv. 209 To repaire the decayes thereof by beheading superstition.
1726 M. Henry Wks. II. 370 It adds to our grief to see a family beheaded.