ISIS hasn't killed any Americans...
...except for a journalist or two. The Americans are leaving Iraq, and their regular armed forces already left. ISIS is killing Syrians (both pro- and anti-Assad), Arabs, Kurds, Christians and Shiites, and even a lot of fellow Sunnis BECAUSE the Americans are leaving. No one has "invaded their homes", they are invading everyone elses homes.
The reason they are able to do this is because we dismantled Saddam's army and our puppet botched the job of rebuilding it, so it can be argued we, the US, are ultimately responsible for ISIS.