What if Spider-Man were a Villain?
As seen in the pages of the Green Goblin fanfic (linked above), Spider-Man was a recurring villain there, and in fact he made a pretty good one. Also in fact, the Marvel heroes could all just as easily have turned out as villains, if you just nudged or tweaked their origins stories a bit. Maybe that's why I always liked the complexity of the Marvel hero.
Case in point, Peter Parker. He already showed his "true" intentions in his real origin story-- he tried to use his powers for fortune and glory on the television, and even made a pretty good sensation judging by a Ditko-penciled montage splash page. But Parker made a decision after that burglar shot his uncle, and the rest is history.
What if Parker made a different decision, however? What if instead of great power, he shucked all responsibility and realized that life should only be lived for himself? Spider-Man has the power and the brains to really make himself into a master criminal!
The trick is to also include Aunt May. They still love each other, and they are all the family they got. Spider-Man will do whatever it takes to make himself *and his Aunt* happy, because he is all she's got. (Yes, in this version, Aunt May is just as "evil," keeping Peter under his thumb with healthy sides of guilt with those wheatcakes.) This adds a little extra to Spider-Man-- he is desperate NOT to get caught, because he wouldn't want the shock to kill his aunt, let alone leave her, well, alone.
Good thing Spidey is so smart; he's got the head of a master criminal with the scope of a petty thief. He only steals what is untraceable and easily converted cash. He'll mislead authorities by targeting areas that are only peripheral to his true targets. He'll always have an exit plan. The worst thing you can do? Corner the guy. He will fight more ruthlessly than you'd believe possible.
This carries over into his public identity, too. Peter Parker is a bit of a jerk and sarcastic mouth-off, but he'll never try to show off (lest he risk exposure.) He'll sell-out his own defeat with pictures to the Daily Bugle, and he'll lie and cheat his way to ensure a date with scores of meaningless one night stands.
In a manner of speaking, the best way to defeat a Flipside Spider-Man is to entice him to become a bigger criminal threat than he already is. It's the only way to make him break the web of security he's built for himself and his persona, and maybe, just maybe, he might slip up.
He's a spider-menace, I tell you!