I am sick to death of the definitional inflation of the word terrorist. Villifying one's enemies goes a long way back, but I think the modern use of terrorist to mean any enemy whatsoever was started by the Nazis, who labelled all resistance to them in the countries they conquered terrorist. Now, they could have called it simply resistance, but such a name would imply that a total project could be resisted. I think that is where the terrorist idea gets its hot air from.
In the case of ISIS, the difference between them and the "moderate rebel" groups the West supports is that they aren't moderate, that they are successful, and that they are gobbling up Iraq. These may well be good reasons for the US and its allies to try to destroy ISIS, War is about this kind of thing. But one must be clear about what is happening. For instance, ISIS is not attacking the US, the US is attacking ISIS. When one reads these panic inducing reports that ISIS may strike at US territory, thus ISIS is a terrorist group, I think: no, ISIS, like any group that is attacked by a state, may attack that state back.
It should be unnecessary to say that the fact that I consider ISIS another paramilitary group does not mean I am somehow for ISIS, or find its beheadings groovy. But I am against linguistic slipperiness which, in the end, has allowed the US for the past couple wars to skirt around the international covenants and treaties it has signed about fighting war. And, indeed, skirt around the constitutional language that sets up strict procedures for warmaking (and which have been ignored by the political establishment since 1945 - if only americans were as strict constructionist about war as they are about owning handguns!).
If ISIS threatens US interests to the extent we have to bomb them, well, lets have a discussion about that. But lets not falsify the discussion from the beginning by pretending the ISIS - unlike, say, the Libyan paramilitaries who we aided in overthrowing Qaddafi - are terrorists. They aren't.