Where are the Teenage Mutant Ninja Turtles when you need them?
Kidding aside, it was a mistake to draw the gun and then a mistake to allow the assailant to close and take the gun.
However, neither are fully indefensible. I've forgotten their names, but in a longshot, if there are credible witnesses to the violent felony behavior by the assailant - in first attacking the turtle nests and then attacking the individual, the lawful citizen with a gun would have the legal right to prevent such actions. I would NOT advise this 'law enforcement' activity because it does put a person in deep and stressful peril, but it can be defensible gray area (with a lot at stake if you lose).
Always ask yourself, as a mental exercise, is my intervention fully necessary and am I prepared to either kill someone or be killed myself, or spend $50,000 on a felony defense trial over this issue? That answer will illuminate the path forward.
A better option here is to stand back, take pictures, and call the police. Let law enforement deal with it and use your pictures and witness testimony as evidence.