Clinging to "decimated" as meaning one-tenth killed is like claiming that "gay" means happy and carefree or that "nice" means subtle or trivial. In any case, the word may have originally had nothing to do with killing or punishing at all, rather meaning "to tithe." When you start to delve a little into the unstoppable evolution of word meanings, you find that word-pedants are often just making fools of themselves.