Are tasers safe?

TASERs have been in the news a lot recently. A student was tasered at John Kerry's speech just a few months ago, and more recently an Oakland cop killed someone with a taser. Then there's the Vancouver guy who was tasered to death in the airport, and in the UK, one man actually burst into flames after he was shot with a taser. He died, too, of course.
Perhaps even more frightening is the Georgia police that unnecessarily tasered a man four times in 40 seconds killing him. Seeing such unnecessary tasering just makes one's stomach churn. Especially when you realize it is everywhere, even when someone is just stopped for a speeding ticket.
The issue, of course, is that because it is considered nonlethal, police seem more apt to utilize it in a situation. But with all these recent deaths, one starts to wonder if it really is nonlethal. That's why the Defence Science and Technology Laboratory (DSTL) in the UK decided to do a study specifically to find out how harmful tasers really are.
Because the cause of death from electric shock generally seems to be a problem with the heart, the researchers focused in on the possibility of cardiac arrhythmia occurring with the use of M26 and X26 commercial tasers. They chose guinea pig hearts to test on, due to the similarity of its electrocardiographic-wave configurations in comparison to human hearts.
The results were surprising for me, though I suppose that's only because I was unfamiliar with similar findings from the past. The current densities of both devices had to be increased by at least a factor of 60 before erratic heartbeats were seen. In other words, tasers are safe when used properly.
There is apparently a wide safety margin between the intensity of a taser strike and the level at which a human heart would beat irregularly. The research team did, however, warn that "consumption of alcohol or some drugs, or an existing heart condition" might "reduce this safety margin in some individuals".
"I have many concerns about TASERs, but the induction of a cardiac arrhythmia appears to be less of a problem," stated Brad Roth, associate professor in the department of physics at Oakland University in Rochester, Michigan. So it remains to be seen why, exactly, so many taser deaths have occurred recently.
Perhaps it just in misusing them that the problems occur? But even if someone is tased four times in forty seconds, how does that compare to the finding that tasers are sixty times too weak to cause irregular heartbeats? These are very unclear questions, and I certainly don't have any answers.
What do you guys think?
Update: PhysicsWorld has an article on today's study, and also I was informed that the UN now considers tasers as torture.