I recently published a post at Echelon Expressions called Why Is The Bad Guy Bad.
There are a lot of possible answers to that question, but here I'd like to address the question of HOW TO make the bad guy bad and do it well.
The bad guys I find most interesting are the ones not entirely bad. I like the ones with weaknesses and vulnerabilities.
Kids agree with me--they love the lonely Frankenstein's monster. They love the henchmen who stumble over their feet. BeBop and Rock Steady--anybody know those names?
A good bad guy may have endearing or laughable flaws--he's vain about his ties or she has a fit if she breaks a nail. A good bad guy may have good qualities--he's intent on destroying our hero, but he'll let an opportunity pass if it would endanger anyone else; she doesn't care if the world goes up in flames, but she'd run into a burning building to rescue a cat or a child. A good bad guy is, above all, a person with many facets to his/her character.
Not that all those facets have to be explored, but a good bad guy's qualities can be alluded to, hinted at, shown in passing or as part of a scene where something of importance to the good guy happens. Anything that lifts your bad guy above the level of Sock Puppet (hi! look at me! i am bad!) is all to the...well...all to the good.