This is the best analysis to date on the web of Jared Loughner's motivations:
"At this early stage, I think Loughner is probably best described as a mentally ill or unstable person who was influenced by the rhetoric and demonizing propaganda around him. Ideology may not explain why he allegedly killed, but it could help explain how he selected his target.
One thing that seems clear is that Giffords, who was terribly wounded but survived, was the nearest and most obvious representative of “the government” that Loughner could find."