Islamic State is a clownishly evil organization. Every day all day, it spends its time beheading, burning, raping and massacring innocent civilians. No group could be more discredited in the larger world than these picture book baddies.
However, since all this carnage is only being done to the other people living in the region, it's really none of our concern. No matter how many people they kill over there, nothing of consequence is lost in the developed, civilized world. For now, sitting back and watching is the reasonable option.
If Islamic State really does become threateningly powerful, nations like Egypt, Turkey, Saudi Arabia, Jordan and Iran should use their own militaries to invade with their own ground forces. These countries have plenty of men and armaments to take out the likes of Islamic State. There is no need for us to go in there and fight their wars.
If Islamic State conquers the entire Middle East/North Africa and then launches a jihad on the West, there's time enough for us to respond only if that happens and when that happens. We can nuke them all to oblivion, or swamp them with tens of millions of soldiers, or whatever it takes, in a worldwide coalition to take them down. No one would accuse us of being the aggressors in that scenario.
The West has stood by and watched as Mao slaughtered tens of millions of Chinese. We stood by and watched when Lenin and Stalin killed tens of millions of Russians. Islamic State is nothing compared to the likes of them. If we're fine watching civilized, good people be systematically liquidated by the millions, surely the offing of a bunch of uncivilized nobodies isn't something we need to get worked up about. Our military is for our own self defense, not the entire world's playtoy.