Extremes on both sides are bad. That being said, there's a real difference. The extreme right is malevolently evil by its very nature. Nazis and fascists are bad because they're evil. They have no good end that justifies the means. They are evil for evil's sake.
The extreme left is horribly wrong. Communists are bad because communism doesn't work. Their actual goal of a stateless, classless society where everyone has everything they need sounds pretty swell, though.
Thus, we have no reason, nor is it wise, to treat both kinds of extremism the same. The evil must be universally condemned, while the wrong must be corrected. One must be shunned, the other engaged. One must be crushed, the other must be shown the facts.
