Q From Richard Nixon: How did spoil (go bad or rotten) come to mean overindulge someone (spoil a child or the kid is spoiled rotten)?
A Both meanings of the word are derived from an older sense of the word in English, which was to strip the armour and weapons from a slain enemy. (This came via French from the Latin word spolium, which originally meant the skin that had been taken from a dead animal. So the first meaning in English was already a figurative one.) From here, the word came to mean the items so removed, booty or plunder, hence our word spoils, as in phrases such as “the spoils of war”. The verb could also be used at one time for seizing goods by violence, to “deprive, despoil, pillage, or rob” as the Oxford English Dictionary graphically puts it. It then took on a less literal meaning of depriving someone of some quality or distinction, and later still to impair or damage something to the extent that it became useless. By the end of the seventeenth century, this had reached the point where to spoil could mean “to injure in respect of character, especially by over-indulgence or undue lenience” and “to become unfit for use; to deteriorate; to go bad, decay”, the two senses you give.