ISIS doesn't represent the whole of Islam
They represent a very literal version which almost no 'moderate' muslims know how to argue against because virtually all modern muslim theology is based on the doctrine of koranic literalism.
Christians and Jews have mostly abandoned literalism*, but muslims haven't and it's why the muslim world lags so badly socially, politically and intellectually.
You can actually plot the decline of Islamic achievement when the doctrine of literalism decisively won a theological struggle. they turned away from science and most have yet to find their way back.
*it's just small freaky subsets, none of which are really mainstream