We have 300,000,000 people in the US. Even supposing that 99.9999 percent of the people are reasonably sane and not dangerous, that still leaves 30,000 dangerous nutters.
When ever a population gets that large, even a tiny percentage of nutters results in a disturbingly large actual number.
I am actually pleasantly surprised that we don't have more of these types of horrible crimes, given our population.