Today, we use decimate to mean destroying the greater part of something. That is a correct usage, except according to the few pedants who still insist on its original meaning.
In Roman times, decimate meant to kill every tenth soldier as a lesson to the remaining soldiers, many of whom were deserting the army. (It strikes me as somewhat counterproductive to kill all those soldiers, thereby ending up with even fewer men in addition to the ones who had already deserted.)
Decimate comes from the Latin word decimare, meaning to destroy or take one tenth. The word for 10 in Latin is decem (with a hard c).
All languages change over time, depending on common usage.