That was very fun Glenn. If another new iPod or iTool comes out in the next month...I'll just have to buy stock in Macintosh.
Warning shots don't exist. Never do them. Not an option unless signaling for help when lost in an abundance of wilderness. Even then, not a great option.
You can shoot someone who is fleeing, as Glenn pointed out. I believe you would have to have witnessed them do an extremely violent crime or murder in order to do so and that you believe they are capable or will repeat their actions. It's a huge if question.
As civilians, I believe witnessing an axe murder would be quite rare, although that would surely be grounds for lethal force.
They should make a tactical iPod. I'm sold on it.