I suspect the mainstream press has a great deal to do with the culture of hate as well, as their bread-and-butter is sensationalism. But then, the press is driven by the tastes of the people, who seem to enjoy World Wide Wrestling and want the general technique to be extended throughout society.
But Hamlet nailed it when he talked about self-righteousness/righteous indignation. The commonality of this seems to be unique to our time and society; while many philosophical and especially religious figures in the past have indulged in it, it never seemed to have infected the regular people so much as now. Or maybe just the records don’t show it as much.
And after all, why not? Is there anything so satisfying as that feeling of “Gotcha!” that tells you that not only are you correct, but you are morally right? And after all, when what a person says is not true, there are only so many reasons why he would be saying it, and none of them are good - either he doesn’t know any better (in which case he’s worthy of contempt) or he’s lying (in which case he’s bad). For the majority of Americans, there are no greys, only black and white. Policians of a certain party have encouraged this attitude significantly over the past fifteen years or so in particular, to the point where one of their nominees for president said rather proudly that he didn’t care to think too deeply about the issues.