Nor are Palin, Beck or Limbaugh responsible. Yes, paranoid schizophrenics do listen to talk radio, and, yes, it does influence them. Palin and Limbaugh, at least, are sane enough to bear some responsibility for their rhetoric. The voices in Loughner's own head, however, would be stronger than their rants.
If I were a theist I would blame this one on God, but I'm not. There are, however, some close human substitutes. They would be NRA supporters and GOP voters.
If the 2004 assault weapons ban were still in place Loughner would still have shot Giffords, but his kill count would be much lower. If Arizona had a robust mental health care system nobody might have died.
People who support the GOP and the NRA are responsible both for the failure to renew the assault weapons ban and for the miserable state of Arizona (and America's) mental health care.
If you are looking for an intelligent and thoughtful response to this tragedy, forget Loughner. Forget (please) Palin, Beck and Limbaugh. Forget gun control, that won't happen in America. Instead, focus on the identification and management of major psychiatric disorders.