I'd disagree that something is art if it's disturbing. Art may be disturbing, but the converse is by no means always true.
I mean, if absolutely nothing else, I am disturbed by quite a few natural ways to die or be injured.
But more generally, what offends people differs from person to person, things that were designed to be shocking 50 years ago barely register now; While things they took for granted are rather disturbing to modern people. Does that mean that what's "art" changes dynamically over time? If so, then why do we have museums and galleries? Why preserve art which will stop being meaningful in short order? Why do we continue to display the works of, say, Michelangelo, since we've lost that entire cultural context, which means that his art must speak less to us than modern stuff?
Just some thoughts.
