They began as bad guys but ended up being good guys ...

Xena (Hercules: The Legendary Journeys)
Callisto (Xena, Warrior Princess)
Angel (Buffy, the Vampire Slayer)
Spike (Angel)

Are there others?

A couple from daytime TV immediately come to mind; Luke Spencer from "General Hospital" and Barnabas Collins from "Dark Shadows" (though Barnabas did slip at times, lol).

I think Sami Brady on Days of Our Lives sort of bounces back and forth between a villain and hero.

Another example is Methos on Highlander.

I think Sami Brady on Days of Our Lives sort of bounces back and forth between a villain and hero.
She's much better as a villain - Alison Sweeney is so good at playing bad!! :D

How about Paris from Gilmore Girls?

She's much better as a villain - Alison Sweeney is so good at playing bad!! :D
She sure is!!!