Then you've misunderstood me. I have never said that ISIS represents all Muslims, nor do I believe that to be the case. It also was not implicit in anything I've said. It is absolutely true that Muslims are the primary victims of ISIS.
Some Muslims commit atrocities, including the killing of innocents. Some of these Muslims do so owing to an affinity for the goals of ISIS, which are explicitly religiously motivated. Others do so owing to an affinity for the goals of Al Qaeda, or Boko Haram, which are also explicitly religiously motivated. Some do so for other reasons. Incidentally, the members of Al Qaeda also condemn and denounce ISIS.
An illustration of what we're talking about here:
I'd ignore the "Salafi" circle as it muddies the basic point (for one thing, all Salafis are Sunni but obviously not all Islamists are Sunni, so not all Islamists can fit under the "Salafi" circle). But ISIS and Al Qaeda will mostly fit under the "Jihadi" circle. They're certainly all Islamists. They are only a subset of a subset of the category that is "all Muslims", but they're still motivated by Islam - a plausible literalist interpretation of religious texts, including the Qur'an but also certain Hadiths they happen to hold as sacred.
Why people seem to think any criticism of religiously motivated terrorism or other horrible actions amounts to painting all Muslims with the same brush, I can't understand. Unless it's just an inability to be able to handle nuance, which seems to be an epidemic these days.