Unnecessary or superfluous. For example: calling an ATM (automated teller machine) an "ATM machine".

You seem to have missed the example I gave.

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.

What would be redundant in this circumstance is saying decimate (e.g. to kill 10 percent of a group) 10 percent of the population. This is of course assuming that the person reading it knows the historical definition of decimate.

Furthermore I used the two different phrasings as examples because if you just wrote "they decimated the population", most people would assume a number larger than 10 percent. But if you try and clarify by stating "they decimated 10 percent of the population", and they know its historical definition, you're being redundant.

So in conclusion, using decimate would either confuse people or be redundant.

As an aside; when you're trying to report something, whether that be a current event or a historical one, you should be using language that the most people will be able to understand for the sake of clarity.