So these people in which their lives we have "destroyed" have decided to destroy the lives of their neighbors for revenge? So if someone came into your house and murdered your family, you would go kill your neighbor for revenge rather than go hunt down those that harmed your loved ones in the first place? Does that make any sense to you?
Do you realize that ISIS originated from the Islamic State of Iraq which was an Islamic militant group that focused more on killing Iraqi civilians than US troops during the occupation of Iraq?
The reason why ISIS was able to gain a significant foothold in regions of Iraq that are dominated by Sunnis was because the Iraqi government under the leadership of Nouri al-Maliki removed many prominent Sunnis from the Iraqi government just short of 2 months after the US military left Iraq in Dec 2011. To make matters worse, Iraqi Security forces had been regularly harassing the Sunni populace as well.
It only took 6 months after the US withdrawal from Iraq for Iraqi Sunni tribes to quickly sympathize with various Sunni Islamic militant groups that began hunting down Iraqi Soldiers and Police.
Obama was trying to say that ISIS is a direct consequence of rushing into the war in Iraq and not managing it properly. The Bush administration was implementing a De-Ba'athification process which would prevent all Saddam loyalists in particular the Ba'ath Party from taking part in the new Iraqi government. Saddam's entire Iraqi Military was also disbanded. As a result, you have tens of thousands of young men who aren't educated and have no job to feed their family. They also don't have any say in their country's new government. This is what caused the sectarian violence that Iraq experienced between 2004-2007. Iraq nearly plunged into a Civil War if the US hadn't realize their early mistake and try to reverse its policy by encouraging the new Iraqi government to allow former Baathists to take part in the governing of Iraq. Consequently, on December 2, 2007, the Sunni Arab Iraqi Accord Front ended their boycott of the Iraq Parliament and on January 20, 2008, Iraq's parliament passed a law to let members of the Ba’ath party to return to public life.
Unfortunately, once the US military left Iraq, for some reason Nouri al-Maliki decided to undo all the progress that had been made by removing Iraqi Sunnis and Baathists from government positions.