"Decimate" originally meant "kill every 10th man"; done by Romans to subdue rebellious colonial elements, &c. But a word's original etymology doesn't always condition the way it becomes used, first perhaps idiomatically [perhaps due to misunderstanding or hyperbole], and eventually as standard., I should say that 'decimate' has probably just about past the idiomatic point of no return, and has acquired the meaning of "kill a great number of" as one of its possible standard usages, and it is in vain to go on being dogmatic about it.