Have you ever read a comic book before, let alone a movie? Almost every issue you've raised is addressed in some story or another from Superman rejecting his power (the world is promptly asked to kneel before Zod). The first thing Peter Parker does is act for self gain and he sees that his family is promptly met with demise. In the watchman, Dr. Manhattan quickly becomes indifferent, while Ozymandias quickly decides that the ends justify the means. Honestly, I think we already know all of the possibilities if we look at all of the alternate universes humanity has scribed that contain such people. In the end it really comes down to the personality of the person wielding the power.
It's really not so much different from becoming a public official. Do you vote to ban cable competitors from your district in return for Comcast financing your re-election? Well, you believe that the health care initiative you're trying to pass is for the greater good so you have to be there to get it through. So you take the money, but then they ask you to sign ACTA. But think of the children without health care. Some people will stick to their virtues and others will fall into corruption. If my both the study and my analogy are correct, then yes, the slide into corruption is slippery indeed.