I'm a really, really evil guy who opposes everything your good guy stands for. I want (the Holy Grail/ world domination/ hidden treasure/ other) so badly that I will do anything, absolutely anything, to get it.
Mr. Good Guy stands in my way, so I have decided to kill him. I've thought about this a lot, and I have a plan to (blow him up, run him off the road/slice off body parts until he's history/other) because I am really, really evil. Once I rid myself of him, I can (live like a king/take over the government/live forever/other) with no one to stop me. (Maniacal laughter here.)
But wait a minute! Mr. Good Guy has a (sister/wife/girlfriend/other) who is really hot. I mean hot! All of a sudden, everything I've worked my whole life to attain doesn't matter so much any more. I want her. In fact, I will allow everything else to slide while my plans for raping this one female take over my universe. And I will follow through, even if (there's a ticking bomb nearby/the police are everywhere/we're more than halfway up K-2 and it's 10 below zero/other). Of course I will not succeed. My lust will bring the end of me and all my plans. Despite all the wicked genius I possess, I'll be brought down because I couldn't keep my zipper up. But hey, it puts sex into the story, and if there's sex, someone will buy it. Right?