It seems like ISIS are one more in a long history of fanatical splinter groups from mainstream Islam and have also pitted themselves against other splinter groups like Al Qaeda, who consider them an enemy.
Maybe ISIS itself will splinter - it's not uncommon for in-fighting within these groups to happen when there is disagreement. The situation is already very messy, with fighting between so many sides, but I can see it getting even worse if more splinter groups set themselves against each other.