Do go read this post by Hilzoy at Obsidian Wings — it’s as clear a statement as I’ve ever seen about why it is objectionable when Erick Erickson vilifies all liberals, or Rush Limbaugh engages in ad hominem attacks against children, or Mark Levin screams that a caller’s spouse should kill himself.
She quotes C.S. Lewis:
Suppose one reads a story of filthy atrocities in the paper. Then suppose that something turns up suggesting that the story might not be quite true, or not quite so bad as it was made out. Is one’s first feeling, ‘Thank God, even they aren’t quite so bad as that,’ or is it a feeling of disappointment, and even a determination to cling to the first story for the sheer pleasure of thinking your enemies are as bad as possible? If it is the second then it is, I am afraid, the first step in a process which, if followed to the end, will make us into devils. You see, one is beginning to wish that black was a little blacker. If we give that wish its head, later on we shall wish to see grey as black, and then to see white itself as black. Finally we shall insist on seeing everything—God and our friends and ourselves included—as bad, and not be able to stop doing it: we shall be fixed for ever in a universe of pure hatred.
Read the whole post.