There are a lot of villains who have crossed the line into heroism - Punisher (when he first started), Venom, Juggernaut, Magneto (for a while) but have there been heroes who turned into villains? All I can think of is Hal to Parallax, but that didn't last long.
Dark Phoenix?
Are we talking Heroes going all the way into villainy or just acting badly for a time and coming to their senses? http://www.superdickery.com/dick/1.html
So, I'll take that as a no. I was talking about crossing over permanently like the way Quicksilver and Scarlet Witch started out with Magneto then crossed over to heroism permanently, with the Avengers. Similarly with Medusa, who's now with the Inhumans. Silver Surfer was kind of a "villain" in his first appearance then became a hero right after that storyarc. But there aren't many instances where a hero crosses over permanently. DP acted badly. Hal went insane for a bit.
John Jameson is the only one I can think of off the top of my head. But he doesn't stay bad either, nor is he consistently superpowered: He's first infected by "Space Spores" starts out as a superhero but then becomes increasingly arrogant and villianous. He is removed of his powers and becomes normal again. Then he becomes the murderous Manwolf... then he becomes normal again and then he becomes Stargod (superhero wolf guy fighting battles on distant worlds). Then he becomes a regular mortal again with no memory of these past glories.