The .22lr is such a "deadly" round because it's an assassins round. It's low caliber means it makes almost no noise when properly silenced. It's small size makes it easily concealable. And it's lack of penatrative power means that if it hits you in the skull, it won't be able to exit the other end, instead it ricochets off your skull and turns your brain to mush.

Not very effective in a war zone or a mass shooting though.

Though now that I think about it, a guy with a silenced .22 subsonic pistol could basically go around shooting people in the head one by one. If they did it right they'd manage to kill quite a few people before anybody notices. That'd actually be way more deadly than some of the mass shootings that have happened so far.