The best starting place is the Social Psychology Network maintained by Scot Plous at Wesleyan University. Another useful place to begin is a site maintained by the University of Wisconsin libraries. For a number of topics relevant to social psychology there are guides to library and Internet resources. As one might expect there are any number of sites devoted to violence in all in manifestations.
Most killers are inarticulate about the motives and contexts of their crimes.
An exception is Clifford Boggess executed in 1998 for two murders committed in 1986. Boggess readily admits to the killings and expresses remorse of sorts, but opinion differ as to whether the remorse is faked or not. For those who want to get their analytic hands dirty the Justice Department publishes a wealth of crime data which can be downloaded and analyzed. Some consider the death penalty to be a state sponsored form of violence, others an appropriate remedy for violent crimes. Of the many sites devoted to opposition to the death penalty perhaps the best is maintained by the American Civil Liberties Union.