To some extent, I say yes, in that I believe that prevalent memes influence people (otherwise why would so many billions of dollars be spent on advertising), and the right has consciously chosen to propagate memes that it is good and necessary to use violence against the left and against the government and against any other opponents. It comes to be accepted as a normal inevitable acceptable part of life. What should be unthinkable becomes possible, in the minds of assholes like the Tucson shooter. Whether or not this crazy shooter was explicitly motivated by Sarah Palin's map or some specific thing is not the issue. The meme's been propagated by the conservatives long before Palin stepped on stage, e.g. by Ann Coulter, a legion of FOX angry crazy talk show hosts, and others. It's impossible to live immersed in US media lately and not be bombarded by ideas like the tree of liberty must be watered by blood, the government wants to install an islamic communist fascist state and must be stopped, blah blah blah, over and over. I'm not even in the US any more and I'm struck by how much I see this theme whenever I read US media, blogs, etc. It's an intentionally cultivated atmosphere of angry violent contempt.
There are other factors of course. E.g. lack of decent mental health care (well, OK, I'll blame the right to a large degree for this for fighting against nationalized health care, not to mention Reagan decades ago cutting funding and putting tons of crazy people out onto the streets homeless), easy access to guns for crazy people (well, OK, I'll blame the right more than the left for this too).
Of course ultimately the shooter himself bears the final blame. But there's something fucked up that this kind of thing happens so much more in the US than in other countries, and it seems clear to me that it's related to things like the intentionally polarized hateful political climate and the gun-obsessed culture.