"barbarous cruelty," late 15c., from French inhumanité (14c.) or directly from Latin inhumanitatem (nominative inhumanitas) "inhuman conduct, savageness; incivility, rudeness," noun of quality from inhumanus "inhuman, savage, cruel" (see inhuman).
And Man, whose heav'n-erected face
The smiles of love adorn,—
Man's inhumanity to man
Makes countless thousands mourn!
[Robert Burns, "Man was Made to Mourn," 1784]
Compare germane, urbane. Meaning "inflicting less pain than something else" is from 1904. Inhuman is its natural opposite. The Royal Humane Society (founded 1774) was originally to rescue drowning persons; such societies had turned to animal care by late 19c.
early 14c., monstre, "malformed animal or human, creature afflicted with a birth defect," from Old French monstre, mostre "monster, monstrosity" (12c.), and directly from Latin monstrum "divine omen (especially one indicating misfortune), portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," a derivative of monere "to remind, bring to (one's) recollection, tell (of); admonish, advise, warn, instruct, teach," from PIE *moneie- "to make think of, remind," suffixed (causative) form of root *men- (1) "to think."
Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended by late 14c. to fabulous animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1520s; sense of "person of inhuman cruelty or wickedness, person regarded with horror because of moral deformity" is from 1550s. As an adjective, "of extraordinary size," from 1837. In Old English, the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression." Monster movie "movie featuring a monster as a leading element," is by 1958 (monster film is from 1941).