I recall reading a news article on a blog of a idiot kid who went around with an unloaded .22 pistol holding people up and doing scare tactics. He approached a guy at a car wash at night and tried it... The guy quickly pulled out his 9mm pistol moved for cover and fired one shot into the teen's chest. The kid's buddies took of running, the teen was pronounced dead at the scene. The gentleman was checked out, license-permit, and cleared by the police. Later the friend corroborated the gentleman's story and police officially ruled it a defensive action.
Now, sad as it is-- I would rather see it turn out this way than the poor guy get unduly harassed or have legal issues because some damn kid wanted to be all big and bad. I believe the police said even threatening someone with an unloaded gun is a threat of lethal force.
As for someone with a tazer, yes. I would shoot someone wielding a tazer. If I had the time I would first draw and make my ability to defend myself known, if he continued to approach I believe I would fire. The reason being that even though the tazer itself is not lethal, I have on me a lethal weapon and who is to say that when I am incapacitated he does not use it on me OR (worse) use the gun for other crimes. Legally I would not be responsible for his actions but ethically I would know that I had a chance to not allow it.