I saw that, too. Being stark naked and all, it was obvious he was unarmed. However, he could have been strung out on PCP, for all the cop knew. Looks like it took 4 or 5 extra guys to help the cop subdue him.
At one point on the subway, the crazy guy is almost entirely naked, but he has his pants down around his ankles.
Obviously unarmed.
Unable to walk.
That's when you move in and use a taser, pepperspray or a good old fashioned tackle.