Are those in ISIS Muslims?

Answer

Ostensibly they are Muslims regardless of the haraam excesses they are committing. However, many of them, mainly those at the top, are atheists or Murtadds. They were members of Saddam Husain's Baa'th Party, and this party consisted of atheists, zindeeqs and murtadds. After America dismantled Saddam Husain's Republican Guard army, many of them joined the variety of Jihaadi groups. They simply started growing beards and shouting Islamic slogans whilst their hearts are still saturated with kufr. It is this kufr which constrains them to act so brutally. They are like the atheists of the Free Syrian Army. These people were originally members of Assad's brutal army of oppressors and barbarians.