Sanders running as an independant would only result in President Trump.
Unless the GOP nominates someone else, by means fair or foul.
Worst case if that happens, Trump drops out and we get GOP candidate X vs a split democratic party.
Best case if that happens, Trump runs as an Independent and we end up with BOTH parties split and a four-way election with two major parties and two independents with strong followings. That would be quiet exciting, to be honest.
That's assuming Bernie runs as an Independent if he doesn't get the DNC nomination, which I hope he does if only to send a message.