I believe the shift in meaning came about because decimation was an incredibly brutal punishment -- it was the most severe sanction that a general could impose.

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.

> 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].

[1] https://www.lexico.com/en/definition/literally

I've heard the term Diezmo to describe the second definition, there's a fiction book about that.

Diezmo (Spanish) and Dizmo (Portuguese) also mean "tithe", an obligatory offering of a tenth of income to churches or religious or charitable organizations. (Growing up in South America in a religious family, I heard both these words a lot.)