What is the percentage? Burden of proof is on you, sir. I haven't personally done studies on the efficacy of tasers, but seeing as simple logic concludes that tasers are used at least, what, 10, 50, 100 times a day across the country and Amnesty International claims a record of 334 deaths due to tasers (these deaths aren't even largely proven attributable to the device) between 2001 and 2008 that the percentage is incredibly small. One study I found quickly of 1201 cases of taser usage resulted in data stating that 99.75% of cases resulted in no to mild harm. Only 2 cases involved death shortly after the taser usage; however the autopsies in both of those cases state that the taser was not the cause of death. Looks pretty non-lethal (in the vast, vast majority of cases) to me.
I would be happy to explain it to the family. I've explained extremely rare, and tragic, outcomes from common actions before. Here's a quick example. 53 year old guy (mostly healthy but hypertensive, obese) had surgery for a ventral hernia .. procedure went completely by the book. No problems, no complications, no abnormalities. Repair was good, decided to keep him overnight for safety's sake, followed standard protocol, etc. Yet, next day, guy gets up to walk around the halls, takes a few steps, and drops dead. Likely PE. Guy died as a result of an incredibly common and statistically benign surgery. Should we call simple hernia repair surgery a "lethal surgery" or "very risky." Nope. Shit happens, but you need to look at the odds. A taser can be lethal, but it is not intended as a lethal weapon, nor does it result in death in any statistically significant way. It's really that easy.