You can see the Latin root for 10 in this word, and originally it meant to slaughter every tenth person. We’ve loosened up a bit since those Roman days and usually use the word to mean widespread damage or destruction.
However, today’s meaning still does not mean total annihilation. Some people or objects have to be left. And don’t use “decimate” with a specific number, as in “Ninety percent of the island was decimated.”