No matter what you might read in the coming days, nobody really knows. Lots of theories, but relatively research on the pathology at play. Here's a 2010 article in the Journal of Police and Criminal Psychology (via the Washington Post):
Incidents of mass murder have gained considerable media attention, but are not well understood in behavioral sciences. Current definitions are weak, and may include politically or ideologically motivated phenomenon. Our current understanding of the phenomenon indicates these incidents are not peculiar to only western cultures, and appear to be increasing.
One obvious possibility is that the actions of one sick individual influence the actions of other sick individuals. Nonstop media coverage - as we're seeing today - might be acting as a sort of stimulant. Whatever the explanation, something is happening. As NYT columnist David Brooks points out, one or two mass killings would occur each decade for much of the 20th century. In the 1980s there were nine; in the 90s, 11. Over the past decade, 27, including today's massacre in Connecticut.
It's probably a mistake to think that we can ever know what "caused" these rampages. But when you read through the assessments that have been done by the F.B.I., the Secret Service and various psychologists, you see certain common motifs. Many of the killers had an exaggerated sense of their own significance, which, they felt, was not properly recognized by the rest of the world. Many suffered a grievous blow to their self-esteem -- a lost job, a divorce or a school failure -- and decided to strike back in some showy way. Many had suffered from severe depression or had attempted suicide. Many lived solitary lives, but most shared their violent fantasies with at least one person before they committed their crimes.