An antagonist can only be considered good if they have something to do for their benefit alone, a motive to do it, and the rescoures to do it. Said thing must be something that can actually be considered a threat to the world. The person doing it must have a reason, such as revenge or desire for power, and the must also be unlike any other character in the game, show, movie, book, etc., the reason being that no one likes a bad guy in said form of media to be boring, and having the antagonist and the protagonist have the same personality is makes things boring. Normally the Antagonist must be a polar opposite to the main protagonist, if only to see the same things in a different way. Said antagonist must also have a particular archtype that sets them appart, the most common of which are Insanity, megalomania, being inredibly self absorbed, or simply believing that the world is dying and by rights should be destroyed.
I think I got everything, but if you think I missed something feel free to throw me a message back.
