I don't think I'd even notice if his safety was on. Even if I did, he could quickly flick it off, or bludgeon me with the gun or attack me with his fists when he realizes his gun isn't working. And sometimes safeties don't work. So he's still an imminent threat to me.
I don't think tampering with his weapon is a good idea, even if you have good intentions in making it "safe" for the responding officers. It might be considered "tampering with a crime scene," or even throw suspicion on you.After making sure he is no longer a threat, and none of his buddies are coming over, I'll secure his weapon- unloaded, slide back, and safed- while calling 911.