One can grab the slide of an semi-auto pistol all one wants. It will still fire. It just won't cycle. The "bad" thing is that it's not a violent thing (holding the slide while someone fires the pistol) so it may not induce the guy to let go. However, someone who is in front of you grabs the slide simply pull towards yourself as you take a step back and, "Voila!" they are in alignment more or less with the muzzle. Let 'em have it.
"Necessity is the plea of every infringement of human freedom. It is the argument of tyrants, it is the creed of slaves." ~ William Pitt, 1783