Today we had news in Ontario that the Police front line is going to be allowed to carry Tasers. I think this is a very bad idea for numerous reasons, which I will state here.
Mis-use of the term "Non-Lethal"
Taser's are mis-advertised to the public as non-lethal. This leads to two issues:
1) When it's proven to be lethal (see http://en.wikipedia.org/wiki/Robert_Dziekański_Taser_incident as an example of a Canadian Taser deployment gone wrong), confusion arises. This confusion is then affects everyone in the chain, from Ministers to the public.
2) When the police think it's non-lethal, they're more likely to deploy it than a normal gun. This means you've got a self-fulfilling prophecy in that more Tasers deployments happen, leading to more deaths.
From a physical viewpoint
If you look at the way a taser deploys it's darts, it's not a side-by-side motion, because in order to complete a circuit, both prongs need to connect with the target. So, it sort of shoots similar to this picture of where you place defibrillator pads.
Now, bearing this in mind, think about the human heart and police in a situation. The target's heart isn't likely to be in a resting state - instead, it's likely to be pumping at quite a fast rate. So take a heart that's pumping hard as a person has been running away, and apply a shock across the chest in this configuration. You're increasingly likely to send the heart even faster, leading to fibrillation. Of course, the officer that just caused this problem is not likely to be equipped with a matching defibrillator.
Of course, all bets are off if the victim has a pacemaker - and this is still a large bone of contention amongst studies.
Risk to the police officer
If the taser connects, the officer is now connected to another person. If you know anything about electricity, you'll know that it can go up wires as well as down them. If the target is wearing appropriate anti-taser equipment, you can discharge a much larger charge from a capacitor back up the wire to the taser.
As an explosives charge.
Tasers are also susceptible to being used as a detonator. So, if a suicide mission has a person wrapping themselves in a layer of steel wool, the police officer might as well be shooting a flame thrower. If the person has more serious stuff on them, the officer is at risk too, as tasers normally require you to be about 15ft from the person.
The drug and alcohol link
Have a look at the statistics on who tasers are used on, and you'll find a majority of people with drug and alcohol usage issues. Unfortunately, this also happens to be the group that are most susceptible to issues arising from being tasered. This disconnect brings us nicely back to the first issue at the top of this article.