Well, think of what ISIS is and does:

It is an enemy of Iran. It counters Shiite influence in Iraq. It destabilized the Assad regime in Syria. Sure, bloody cutthroats, rapists, murderers, but Saudi Arabia likes them, because Iran. They will likely help Saudi in Yemen as well. ISIS is today what AQ was back in the 70s and 80s.

Who said that whenever two enemies are fighting each other, you should help both sides?

And Americans wonder why the rest of the world wants to destroy them.

Edit: Pointing out that I am an American.