They want people to pick sides. To force the issue. It seems like they want to draw the West into further meddling so that they can divide the region even further. More American soldiers in Syria/Iraq/Libya will give them more ammunition in their ideological justification and thereby swell their ranks and thus give them more power. All while conveniently terrorizing or exterminating the minorities they want gone anyway. Eventually they can dial down the violence and the West will withdraw and then they will be left with the regional influence. This is the same playbook that was used during the sectarian struggles in Iraq in the mid 2000's. ISIS is just putting it on TV and being as gruesome as they can for maximum attention. Remember, the first mentions of ISIS were not about their barbarity, they were about their slick videos and dramatic footage. ISIS is all about the ratings. And they know what sells.

/u/demosthemes put it fucking perfectly in another comment in this thread:

They want people to pick sides. To force the issue. It seems like they want to draw the West into further meddling so that they can divide the region even further. More American soldiers in Syria/Iraq/Libya will give them more ammunition in their ideological justification and thereby swell their ranks and thus give them more power. All while conveniently terrorizing or exterminating the minorities they want gone anyway. Eventually they can dial down the violence and the West will withdraw and then they will be left with the regional influence.

This is the same playbook that was used during the sectarian struggles in Iraq in the mid 2000's. ISIS is just putting it on TV and being as gruesome as they can for maximum attention. Remember, the first mentions of ISIS were not about their barbarity, they were about their slick videos and dramatic footage. ISIS is all about the ratings. And they know what sells.