From Old English *cyllan, from PIE *gwelH- (to pierce) — its doublet 'quell' softened from 'kill' to 'suppress.'
To cause the death of a living organism; to put to death.
From Middle English 'killen' or 'cullen' meaning 'to strike, beat, kill,' probably from Old English *cyllan (unattested but inferred), a causative form related to 'cwellan' meaning 'to kill, to torment.' Old English 'cwellan' came from Proto-Germanic *kwaljaną (to torment, to kill), from PIE root *gʷelH- meaning 'to pierce, to sting, to pain.' The word 'quell,' which now means 'to suppress,' is the direct descendant of 'cwellan' and originally meant 'to kill' — the same verb, differently evolved. Key roots: *gʷelH- (Proto-Indo-European: "to pierce, sting, pain").