I think it's slightly more pathological than that. They are so convinced of their own rightness, that anyone who opposes them must surely be totally wrong. So wrong, in fact, that the left can do anything they want to try to silence or neutralise them. An enemy of the "looney-left" is so clearly wrong and bad in their eyes, that they can be lied about, falsely accused, attacked, and so on, so sure are the left of their position as the good guys.