It works on so many levels because, when you are speaking to someone who is both a douchebag and a retard, you certainly don't want to lose any more time than you have to.
As an added bonus for montrealers, douchetard is close enough to Couchetard that you'll say it without even noticing.
Alice: Derek, you're such a fucking douchetard.
(i.e., an asshole, jerk, jackass) and a "retard." Expresses both unpleasantness of personality AND stupidity at once.