Apparently "decimate" is having a moment on HackerNews lately. It came up over the weekend as well [1], with the same disagreement between people on what it actually means.
> dec·i·mate /ˈdesəˌmāt/ verb 1. kill, destroy, or remove a large percentage or part of. "the project would decimate the fragile wetland wilderness" 2. HISTORICAL kill one in every ten of (a group of soldiers or others) as a punishment for the whole group.
There are many words and phrases that have changed in meaning over time through popular (mis)use.
What is often overlooked is not just that it was "one in ten" executed, but that they were executed by being clubbed to death by the other nine men who were not chosen to die.
So "decimation" spread from the literal procedure to "worst outcome imaginable". Much as "literal" no longer means literal.
To be clear, "literally" still means the original definition, but now there is an additional colloquial definition which can be used as the opposite of the original meaning[1].