Perhaps tasers are lethal to some people who have nerve issues or who are epileptic. There are people who have bad heart conditions and a taser could trigger a heart attack and since the people who are using the tasers are usually not doctors, there is no way of knowing for sure if that person will survive.
Tasers only kill those people who already have underlying health issues when they are stunned. In general, tasers are not lethal weapons because they are simply designed to paralyze victims temporarily so they stop what they are doing. Deaths incurred by tasers are rare compared to those who die from actual gunshot wounds.
It is true that people have died after having been 'tasered'. In nearly every such case were there was a fatality resulting from someone being 'tasered', there was drugs involved or the person had some underlying condition. When healthy, sober people are tasered there usually is no long term effects and rarely death.
No, tasers are not actually a lethal weapon, because they have not actually ever killed someone. There are people who claim it, but they only reason that they do so is because they want to sue law enforcement and collect a large reward. Tasers are an effective law enforcement tool.