We may have some clues to Loughner's motivation, but nothing definitive, since he hasn't said anything and the feds are probably withholding the most relevant material they may have found in his home (especially the note found in his safe).
So, we're walking in darkness here--and dealing in our own personal punditorial prejudices. Given what we do know, the notion that Sarah Palin's "reload" target ads had anything to do with this barbarity has less likelihood than a scenario that both Andrew Sullivan and I would find terribly inconvenient: that Loughner was a paranoid schizophrenic whose illness was exacerbated by frequent marijuana use. (Both Andrew and I favor legalization.) I'm not saying that marijuana was the cause of this--again, no one knows why Loughner went berserk--but as Massimo Calabresi writes below, frequent drug use can have a deleterious effect on those with schizophrenic tendency.