It's a bit more nuanced than that. Apparently, Otto Octavius (a.k.a. Doctor Octopus) was dying, so he engineered some scheme to switch brains with Peter Parker. Peter, in Doctor Octopus's body, dies, but somehow Doctor Octopus, in Peter's body, suddenly gains access to all of Peter's memories, showing that the brain transfer was not complete. Seeing Peter's whole life in that light, Doctor Octopus was inspired, and instead of using Spiderman's body for evil, he vows to not only fight crime, but to do a better job of it than Peter ever did, thus becoming the "superior" Spiderman.

Now, I'm not abundantly familiar with comics in general or Spiderman specifically, but I know that in comic books, few things are quite as temporary as the death of a superhero. This one example has a particularly easy retcon capability. After everyone gets tired of seeing Doctor Octopus trying to be Spiderman (which sounds like a pretty interesting story premise, actually), they could just say, "oh, Peter Parker's original memories and personality completely regenerated and deleted the Doctor Octopus persona from his brain," thus effectively returning Peter Parker to the role of Spider Man once again.

To tell you the truth, I was more offended by that one time they decided to let Spiderman be a black kid for a while. Like all such gimmickery, it was just a bizzarre plot twist to raise interest and sales, but this time they did it under the banner of racial equality. I found such a sentiment to be utterly false and pretensious. Of course they weren't going to let Spiderman be black forever. I knew, sure as anything, that before long Peter Parker would come back to life and we'd never hear from that black kid again. With that in mind, for them to suggest that they were being "progressive" or whatever came across as apallingly insincere. By the way, does anybody know how they retconned that? I remember hearing about it, but I never heard how they changed it back.