The phenomenon fascinates me almost as much as it horrifies me. I was a Protestant myself until three years ago, and I had NO idea how much hatred there was for the Catholic Church and for Catholics. I never heard anything as vicious as I read on FR on a daily basis. When I was growing up, my devout (and very well-read) Protestant mother just explained that Catholics were Christians like us, but believed slightly different things; the point, she said, was “believe in the Lord Jesus Christ and ye shall be saved,” and clearly Catholics too believed in Christ, so no harm, no foul.
It comes from their tradition. The Reformation was like a divorce, where each of the parties must persuade themselves how awful the other is. In the worse cases, of course, it is more like a murder.