"mania for murder or killing," 1842, from Greek phonē, phonos "slaughter, bloodshed, killing" + mania.
1832, Latin, literally "body of the offense;" not "the murder victim's body," but the basic elements that make up a crime, which in the case of a murder includes the body of the victim. For first element, see corpus. With delictum "a fault, offense, crime, transgression," etymologically "a falling short" of the standard of law, neuter singular of past participle of delinquere "to fail; be wanting, fall short; offend" (see delinquent).
Thus, a man who is proved to have clandestinely buried a dead body, no matter how suspicious the circumstances, cannot thereby be convicted of murder, without proof of the corpus delicti--that is, the fact that death was feloniously produced by him. [Century Dictionary]
c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of a cattle or sheep for food;" from a Scandinavian source akin to Old Norse slatr "a butchering, butcher meat," slatra "to slaughter," slattr "a mowing," related to Old Norse sla "to strike," from Proto-Germanic *slagan- (see slay (v.)).
The form was perhaps influenced by Middle English slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing."
"to kill (many beings) indiscriminately," commonly in reference to those who are not in a condition to defend themselves, 1580s, from French massacrer "to slaughter" (16c.), from massacre (n.) "wholesale slaughter, carnage" (see massacre (n.)). Sometimes 17c.-18c. merely "to murder cruelly," without reference to number. Related: Massacred; massacring.
1560s, "pertaining to or consisting of a rite or rites," from French ritual or directly from Latin ritualis "relating to (religious) rites," from ritus "religious observance or ceremony, custom, usage," (see rite). By 1630s as "done as or in the manner of a rite" (as in ritual murder, attested by 1896). Related: Ritually.
Middle English, from Old English cwealm, cwelm (West Saxon) "death, murder, slaughter; disaster; widespread death by plague, pestilence or illness affecting humans or livestock; torment," utcualm (Anglian) "utter destruction," probably related to cwellan "to kill, murder, execute," cwelan "to die" (see quell).
The sense softened to "feeling of faintness" (1520s); the figurative meaning "uneasiness, doubt" is from 1550s; that of "a scruple of conscience" is from 1640s.
Evidence of a direct path from the Old English and Middle English senses (now obsolete) to the modern senses is wanting (OED 2nd edition has them as separate entries), and the old word seems to have become rare after c. 1400. But it is plausible, via the notion of "fit of sickness." The other suggested etymology, less satisfying, is to take the "fit of uneasiness" sense from Dutch kwalm "steam, vapor, mist" (cognate with German Qualm "smoke, vapor, stupor"), which also might be ultimately from the same Germanic root as quell.