Assad’s partner, Iran, is as big a terrorist supporter as any who support ISIS.
Assad and Russia did not defeat ISIS. The Kurds did in Northern Syria (with the help of the USA).
Those are facts.
The Middle East is a complex place, Assad is on the wrong side in the overall battle.
ISIS sprang up and took root in places where the U.S. destabilized and/or toppled a sovereign government that was perfectly capable of dealing with radical Islamic elements within its borders. They also flourished with the tacit support of the U.S. government in these places.