Liberals hate themselves. Deep down, they know that they are self-centered, neurotic, emotionally-stunted children who don't really care about anyone else.
And, if they are that messed up, so they think, their ideological enemies (i.e. Conservatives) must be downright evil! After all, if Liberals are the good guys and they're that screwed up, Conservatives must be psychotic!
It never occurs to them that maybe it's just them.