The premise is simple: there's a process or substance that grants superhuman abilities. The catch is the abilities are not permanent. You need a regular supply of the stuff. If it involves gadgets then they break down or burn out or run out of whatever exotic fuel they use.
Fortunately you have a supplier who wishes to remain unknown. Everyday you receive your new dose/batteries etc in the mail or at a pick up point. There seems to be no permanent side effects from this. What you do with the power is up to you.
Choice Number 1: You use the power to embark on a one man crime wave.
Result: The supply stops. Period. In fact you might just run out of juice a few hours early when you are flying across town or lifting a ten ton safe if you go for such activities.
Choice Number 2: You decide to use the power for you own benefit, not necessarily robbing or looting but giving yourself an edge. You go to Las Vegas to clean up or enter professional sports etc.
Result: The supply stops. Period. You probably aren't flying or throwing cars around when it wears off. You didn't make any rivals or enemies while you were an overnight sensation did you? Those you keep.
Choice Number 3: You use the power to make the world a better place. Maybe you wreak havoc on the forces of evil (spandex and mask optional.) Maybe you low key it and work your miracles in street clothes. The styles vary.
Result: The supply continues. In fact you may get more potent powers over time as the supplier feels they can trust you. The status quo is maintained for a while. Then the supplier informs you that for the arrangement to continue you have to commit a crime. Something little like stealing a car say. If you refuse the supply stops and you go back to a life of quiet desperation. Every time a disaster or crime occurs that you could have stopped you die a little inside.
If you agree to performing the task (it's only a car c'mon) the supply continues and may even be increased. Then a few days or weeks later the supplier wants you to do something else.
The crimes continue increasing in severity. Robbery, assault, battery, and finally murder. the question is at what point do you call it quits? Or do you descend into denial and rationalizing to justify the crimes you commit in the name of justice?
The edge doesn't have to be super powers per se. In Traveller a suit of high tech battledress with a grav belt might make you a super human effectively on a low tech world. Bullets would bounce off you. You could fly but you might not be able to recharge it or activate it without a password that changes everyday.
For added paranoia have several people in the same situation meet up and form a team. Perhaps a world of supers all have this origin and all preform darks deeds at times never suspecting they are not unique.
Who the supplier is and their goals is up to the individual game master. Maybe it's a test and refusing to commit a crime results in a new supply of upgraded powers after a few weeks. The supply was looking for a truly worthy soul.
Maybe the process does have long term effects: mental instability and suggestibility and the suppliers is running a huge experiment to refine the procedures.
Maybe it's all a sick and twisted game. See how far you can get a person to go to remain superhuman. Last person whose hero goes psycho wins a coke.