If it were phrased “they decimated the population” most would assume from the phrasing that it mean that you’re saying that a large proportion was killed, because that’s how that word is actually used in the English language. If it were phrased “they decimated 10 percent of the population” you’re either using the word as people understand it wrong or your saying they killed 10 percent of the population twice right next to each other, which is you know, redundant.

The definition of words reflect how we use them. An interesting fact is that scientists use Latin for scientific names of things because no one speaks Latin so the meanings of those words will not change with time. It’s the same in courts, you’ll find that a lot of old English words that aren’t commonly used in everyday conversation are used and that’s so that the meaning of things stay consistent over time.

Most people don’t know the historical definition of decimate, so using here it would be confusing or redundant.