I had to do some research. It is hard over there to determine who are the good guys when they are mostly bad guys!
ISIS is comprised of radical Sunni’s. Many (all?) of the leaders in ISIS are former Saddam Hussein’s generals, leaders iirc.
Iran is primarily Shia. Saudi Arabia is mostly Sunni.
It is true that most of the groups fighting it out in Iraq are anti-American (Kurds are a notable exception).
Many of these groups are anti-American primarily because they want absolute power for themselves, not some bunch of checks and balances on their power.
The one thing I would elaborate on from your post, is the role in ISIS leadership of former military from Saddam’s Ba’ath Party.
The top leadership of ISIS is from the old al Quaeda in Iraq. They have incorporated several former Ba’athist military into their military command from meeting in prison, from religious recruitment, and from tribal connections; which has helped them be more successful militarily.
Also, the main Ba’athist rebel groups, notably the Naqshbandi Army (http://en.wikipedia.org/wiki/Army_of_the_Men_of_the_Naqshbandi_Order), collaborated with ISIS even though they ultimately don’t support them, mainly because they collaborate with anyone working to take down the government. They plan to take over in the Chaos, and to draw support by being seen as the technocrats who can bring order and make the trains run on time. The Naqshbandi Army is based on Ba’athism and Sufi Islam, which although Sunni, is opposed to the fundamentalist Wahhabi-style Islam of ISIS. They are also down on the Shia.
Other Sunni groups and tribes also aided the ISIS incursion because they wanted to get rid of the oppressively Shia government of PM al Maliki. Many of them are regretting that now. A lot thought that they could use ISIS to get rid of the Shia, and overthrow ISIS later. ISIS proved to be a lot better prepared to brutally squash opposition than they expected.
More of the complex mix of bad actors and changing alliances that make the Middle East so “interesting”.