He wants power and is not afraid to use sex to get it. Sometimes he keeps his fly unzipped as a way to get easy access to police files and lawyers' offices. Other times he is hiding something behind him and attempts to distract the heroine by standing in front of it and lowering his pants suggestively while smiling and raising an eyebrow. If he is being interrogated, there's no need for him to invoke his Fifth Amendment rights or even lie; he just wears really short shorts without any underwear, and when the cops are in the middle of interrogating him, he leans back, opens his legs, and lets his testicles fall sexily out of one leg hole.
If your show or movie has supernatural themes, you can use demonic possession or some other spell or curse to transform a formerly "good boy" character into an homme fatale. No matter what the character was like before, or how many similar transformations have come before it, you can always show demonic possession thusly: Make the possessed man pout seductively at the camera and then jump on the nearest character, male or female, and rub himself against her while talking about how happy he is that he is finally free to be a bad boy. If the possessed man has a chance to change his clothes after he turns evil, make sure his new clothes are far more revealing and show off the lines of his crotch.