You can use deadly force to stop a violent felony. Strapping random people in your presence is a violent felony. A man in close proximity with a knife is just as deadly as a man with a gun.
For the record, I would also have ordered him to put down the weapon and held him for the police. IF I could do so with out putting myself or others in jeopardy. Other wise I would have shot on the spot.
Strapping = Stabbing.