Here's an article from Massad Ayoob I just saw about the incident in question. Apparently the investigators took the citizen's side.
I can only repeat what I was told by my CCW instructor (his credentials are in my previous post.) I also assume you intend to ask your question in the context of in defense of another in a public setting.
In Illinois, unless I, or someone I am immediately with are threatened with direct harm out in public, I should not pull my weapon and intervene. My responsibility under the law as my instructor understood it, was to remove myself and whoever I was immediately with away from the scene.
That is unless I want to be the first test case in Illinois for coming to someones defense and using my weapon (presumably to injure/kill/fend off the attacker of another individual.)