The numerator of psycho killers like Jared Loughner doesn't appear to be large, and it may be shrinking due to better medications and the like. This story is big news for a bunch of reasons (e.g., the press wants to use it to launch Obama's re-election campaign the way Timothy McVeigh launched Clinton's re-election in 1995), one of which is that it's a man-bites-dog story: Members of Congress and judges don't get shot very often at all.
P.S. Jerry Pournelle writes:
Allowing the non-violent madmen to live among us is a price of liberty; and allowing physicians and police to lock people away because they are mad is conceding a power to the authorities that often proves unwise, and sometimes is simply an adjunct to tyranny.