It wouldn't surprise me at all if the .22 has killed more people than any other cartridge. I'm not saying it has, but it wouldn't surprise me. That's because its death toll would have to include:
1. deliberate use for self-defense, including when the .22 was used because nothing else was available. Taking into consideration that the .22 rimfire is the most common cartridge out there, and (at least in .22 Short form) has been around longer than any other cartridge still available.
2. Playing around and getting stupid with guns, when the .22 is also most likely to be used by beginners.
3. Didn't know it was loaded, see above.
Whether it's any good for self defense is moot when trying to decide if the .22 will kill, because the point of self defense is not killing; it is to stop the other guy RIGHT NOW. If the bad guy drops dead from the .22 shot but only after he's taken the gun from you and beaten you to death with it, he's still dead and the .22 is therefore still, by definition, deadly; but it sure wasn't a very good defense weapon!
I suppose fast cars and cigarettes are even deadlier, 'tho I haven't thought on trying to use them as defensive weapons. I'm trying to figure how that would work. "Here, smoke two packs of these a day for twenty years and come back to finish your robbery then." I don't think that's going to fly.