Are ISIS soldiers summarily executed by IRan, Russia, Assad and Iraq?

Yes. This is likely now. ISIS has not shown mercy to anyone and so no mercy at all will likely be shown ISIS soldiers from now on either, especially by Iran, Russia, Assad and Iraq who are Anti-ISIS and for the extinction of anyone and anything connected to ISIS.

This is actually logical because Terrorists and terrorist organizations do not fall under the Geneva convention. To become a terrorist is to completely lose all rights as a human being worldwide and to be treated more than likely more like an animal than a human by most nations on earth. This might be different if someone is brought to trial. But, in a Middle eastern country people who are thought to be terrorists have no rights at all, especially to Russians, Syrians, Iraqi, and Iranians.

However, then "How does each country define a terrorist?" For example, to an Iranian in Iran any American might be considered to be a terrorist there.