A taser can certainly kill someone - depends on all sorts of things. It takes very little amperage to interfere with the hearthbeat, but it has to be in the right place. People with implanted pacemakers and the like are definitely going to be vulnerable depending on where you hit them. There have also been quite a few deaths in the US where, like the Canadian guy, there is no other explanation.

(some people survive lightning strikes that conduct through them so you can't say tasers always kill. Then again, lightning strike victims often die too)

Anyone using a taser needs to keep in mind it might well be fatal, and you don't know ahead of time if the target has a defibrillator implant, etc. When it's needed fine - it's less risk of death than a .45! - but keep in mind there is a risk. "Backtalk" is not a good reason.

And use the time to escape! If someone uses such a weapon on me you can bet that if I don't see a police uniform after I recover I will use all means available to defend myself against another attack...