I'd wait until shots were fired or until it was painfully obvious (to everyone) that someone was about to be injured or killed. I would make sure that I was in a situation where a no-bill from the grand jury was a cake-walk. With my luck, if I acted sooner, under the ski mask it would be some 15 yr old kid with a mail order replica gun.
A kid in a ski mask with a replica gun has a clear intent of making others fear for their lives.
Usually, putting someone in fear for their life makes them do what you want them to do. Other times, among a small percentage of people in nearly all the states, putting someone in fear for their life will get you injured or killed. If a 15-year-old kid is hard-core enough to go out making people fear for their lives, he's hard-core enough to take a bullet or two to center mass.
I wouldn't feel too bad about someone shooting him if at the moment they pulled the trigger they thought that it was a real gun, regardless of whether it turned out to be fake.