The majority of people that ISIS kill are Muslim.
Their extreme interpretation of the Quran calls for apostates (interpreted as any Muslim who doesn't follow their strict code) to be murdered, the Saudi monarchy and anybody who supports them, then thirdly the Crusaders (the West) who they view as wanting to crush Islam. Anybody who is left is seen as fair game for either slavery (men and children) or concubines (women)
As barbaric as they are, their extreme dogma should be their undoing, because ultimately the califate needs to expand into other territories. They have only gained so much influence because of the vacuum left in Iraq and the civil war in Syria. If they tried to expand into Iran, Jordan, Turkey or Saudi Arabia, they would be crushed.
Also the Califate demands that Muslims go there to become part of the Islamic State, not go to fight overseas. The doctrine is to create an Islamic State, draw in the Army of Rome (the west) defeat them and trigger the Apocalypse (these are fundamentalists after all) and create a new Islamic world. As much as ISIS like to claim responsibility for Paris, it doesn't fit with their ideology, that attack was much more likely to be Al-Qaeda who have carried out similar attacks and who's doctrine is congruent with causing terror on foreign soil.