Speaker's position is the "right", "correct", "best" position, etc.
Furthermore, the goodness of the speaker's position should be evident to anyone with half a brain.
To actively oppose that which is good makes one EvIl?.
Due to the manifest and obvious goodness of the speaker's position found in premise #2--anyone who believes that another position is better must be stupid.
To oppose the speaker's position therefore means that a) the opponent is opposing that which is good; or b) is too dumb to tell the difference
Opponents of the speaker's position are either evil or stupid. [Usually unspoken follow-on: Which one are you?]
This arguments is fallacious because either of the first two premises (on which the whole argument is based) could be wrong -- either the speaker's position could be incorrect, or there could be reasonable disagreement on the question; or there could be no clear way to discern what is best. Usually, users of EvilOrStupid want to deflect attention from these deficiencies; and therefore use EvilOrStupid to put the opponent on the defensive.
EvilOrStupid is a type of AdHominem attack; as it shifts the topic of argument from the question at hand (the merits of some X) to the merits of the opponent.
One clever retort to EvilOrStupid is to note the following:
A person who is both evil and stupid will a) wish to oppose that which is good, and b) not be able to tell the difference.
Such a person will conclude that !X (where X is the speaker's position) is the best one (because he is stupid), and therefore support !!X (because he is evil).
A person who is both EvilAndStupid? might well agree with the speaker!
Obviously, this argument is as fallacious (if not more so) than the EvilOrStupid argument above. But it's amusing nonetheless...
It's reminiscent of a scene from ThePrincessBride called the BattleOfWits.
See: FallaciousArgument, FalseDichotomy