Depends on your perspective, as you are facing someone who does not care about the law or YOUR life. I would rather keep on living at the expense of some scumbag assaulting me.
This is the scumbag assaulting you with his 'life-saving device', yes?
I don't have a problem with legitimate self-defence, but as a pedant I can't accept that a lethal weapon is a 'life-saving device' either. A pacemaker is a life-saving device, as is an aircraft ejector seat or an air-bag in a car.
But claiming that a device designed to injure, maim or kill is a 'life-saving device' is pushing a point too far for me.
Guns are life-saving devices...
Er, I think you might have got that wrong!
Why isn't just not using one at all an option?
One man's constant is another man's variable. -- A.J. Perlis