Always good to remember that in self defense handguns you go to the bigger calibers for more STOPPING power, not more KILLING power.
.22 will definitely kill, and in fact (as somebody else has pointed out here, I forget who, sorry) it was (or is?) a preferred assassination weapon of the Mossad, among others.
It's just that the fact the adrenaline-pumped sleazeball coming at you is going to die three minutes after you shoot him doesn't give much comfort if he stays on his feet long enough to crush your skull with a tire iron first. Killing isn't the issue, slamming him hard enough to make him go down NOW is the issue.
I wouldn't be surprised if the .22 had killed more people than anything else just because it's so common, so often used by beginners, and so underestimated. But it's deadly enough, for sure. As far as that goes, around this town a few years ago the Conservation Officers searched out a local kid who killed a whitetail buck with a .177 pellet gun. If it looks like a gun and acts like a gun, you have to treat it with the respect of a gun. Except maybe soft-air, and perhaps somebody will pop up here with a horror story about THAT.