Depends how you define terrorism I guess. Certainly the 9 million they killed in the holocaust weren't exactly combatants. I would define them as so but I know people in the West especially seem to have a large problem referring to governments, no matter how bad, as terrorists, so I guess a lot of people wouldn't agree.
The difference here is ISIS is not a state actor but a militia as you already outlined yourself and certainly at least a significant number of them are not Syrian or Iraqi. And many of their tactics are to strike immense fear into those they're fighting/ looking to subjugate.