American politics has always been filled with rough rhetoric (I'm an historian, and I can state that 19th century political discourse was quite bombastic). But the rhetoric in the last few years has used words that are "over the top" inflammatory. And ss the Arizona public officials have been saying, "words are important." Just as those groups who picketed and singled out abortion providers did not advocate (at least openly) killing them, Palin and the far right types I am sure did not think that their words would elicit yesterday's killings. But, words are important--and people who are really crazy have access to automatic weapons and can carry through with what they believe those with inflammatory rhetoric "really" want.
Frankly, the press is another party that bears the burden of this tragedy. The American press has always been a "for profit" enterprise, but at least in the 20th century, there was a sense they had a "public" obligation, even as profits were important. But with profits now the most important thing (and shareholders who demand good returns)the press has every reason to do whatever it takes to get a story that will attract readers/listeners/watchers. They are more often irresponsible in reporting over-heated rhetoric from people that would have been ignored years ago. What will become of American democracy when elected representatives are too scared to meet face-to-face with their constituents?