...and x1000 times that number of people were killed with kitchen knives, claw hammers and bare fucking hands in the same period of time. By your underaged logic, that makes those "weapons" even more effective than a .22. So you should throw even the .22 away and carry a fucking hammer instead, because hammers killed 50000 people worldwide last year. See where this is going?
The reason serial killers often used .22 was the same one it is still popular in the ghetto, along with .38 revolvers despite being obsolete as a combat weapon back when it was made. It's cheap and ubiquitous, especially back in 1970's when most of the famous spree killings happened. A .22 revolver was probably the cheapest type of handgun you could buy. And because it was so cheap it was also very common - it's only now that it's relegated to fudds and ghettos. This ubiquity made them be stolen and/or appear on black market more often, making 22's even more attractive for criminals.
Most violent criminals are fucking retarded, they know less about guns than the dumbest fudd and they are broke as well. So that's why they buy 22's.
Of course it can still kill. So can an air rifle. Or a knife. Or a chair. Or a rock. Or a fucking Aluminum tea spoon. But that doesn't make them a good weapon when so many better ones are available.
And no, .22 has literally nothing in common with .223. 223 hits like a freight train, piecing 3A body armor and blowing chunks of meat out of people at 100 yards. It's a rifle round. Google Kyle Rittenhouse shooting, that's one of the recent good ones.