Geopolitics aside, some would say that a strong man who sees a horrific crime that he has the strength to stop has the moral responsibility to do so. ISIS has conquered territory by force of arms - do we want to allow that sort of thing to be acceptable on the world stage again? The way ISIS is treating their conquered subjects is horrific and appalling, and we should probably put a stop to it.
Some would also suggest that the current instability within the Near and Middle East is the result of European colonial powers drawing national borders in such a way to cause instability and invoke inter-racial and inter-religious tension.
Perhaps the better solution is to withdraw from the area and let the regional powers work the issue themselves. If that means a century of warfare, not unlike what Europe experienced after the Protestant Reformation, then so be it.
Sure, such a conflict would result in a spike in the price of oil. But last time oil went above $160/bbl, we saw factories in North America being brought out of mothball, a renewed interest in alternative fuels (methane, nuclear, solar, wind), higher urban growth, an increase in the use of transit and a decrease in the use of petroleum derived fertilizers. Our economy and environment actually benefit in many ways when oil gets expensive.