What on earth is one of New York's finest doing carrying a knife in a bar on a night out, and using it to stab people. Why hasn't he been arrested? In a case of a violent dispute between an off-duty cop and dark skinned foreigners is the default position that the cop must be in the right?
We do not know what type of knife it was and, in NYC, carrying a concealed knife (provided it meets certain criteria) is not necessarily unlawful.
In NYC, as in the UK, carrying a knife, of any type, for 'self defence' is unlawful. If, however, you are carrying a 'legal' knife for some other purpose (e.g. a Swiss Army knife, which comes in handy for all sorts of mundane routine tasks) and you happen to get attacked and use the knife in self defence, then that would probably be lawful. As a cop, presumably, he would have been careful to ensure the knife in question and the reason for his carrying it in a public place, were lawful. We'll see.