The thing about Tazer use killing people is sort of a red herring. Yes, there are people who die after being Tazed. There are also people who die while being restrained who weren't Tazed.
Some die due to a phenomenon called "positional asphyxia," some die due to the lethal arrhythmias caused by drugs and/or alcohol. Each year a small number of people die suddenly while restrained. Most of these deaths are associated with individuals who were restrained while being taken into custody during a violent police encounter. Other cases of sudden restraint death involve individuals in detention or residential treatment programs who were restrained during violent encounters while also under the influence of psychiatric medications.
No one is certain how many restraint related sudden deaths occur each year. Identifying the exact cause of death is the biggest problem. The number of estimated deaths are in question but may range between 50 and 125 per year. Some estimates are higher.
Sudden death after individuals were taken into police custody has been reported for several decades (yes, even BEFORE Tazers were invented). The term "in-custody" death syndrome was first used to describe unexplained deaths when there was no apparent cause other than a police arrest. The main features of this extreme state included agitation, excitability, paranoia, aggression, great strength and non-response to pain. Such behavior is now referred to as "acute behavioral disturbance". The term 'acute" refers to a rapid onset (hours) and not gradual onset (weeks) of this behavior.
The real question is what was the in custody death rate before Tazers were employed, and what is the rate now. In actuality, the death and injury rates of both suspects AND police have fallen since Tazers became widely used.