Raistlin Majere
Well you could argue, that he isn't a villain all the time... Or that this all was reverted, but still!
edit: Since I don't think he is well known, he is a character in the dragonlance universe.
Read Dragonlance Chronicles and Dragonlance Legends.

Well because of limited time a very short version:
Even so he was part of the heros group he joined forces with Takhisis the godess of evil.
He betrayed Takhisis to gain a chance to kill the only mage who could possibly be more powerfull than him.
"Later" he lured Takhisis into the real world and killed her, gaining her place as god.
He then proceeds to kill every single god, lay waste to the world and ultimately ending it.
All while keeping his former teacher (I don't know a better word here...) alive to witness the destruction of the world so he would have his revenge on him.
Even so he was part of the heros group he joined forces with Takhisis the godess of evil.
He betrayed Takhisis to gain a chance to kill the only mage who could possibly be more powerfull than him.
"Later" he lured Takhisis into the real world and killed her, gaining her place as god.
He then proceeds to kill every single god, lay waste to the world and ultimately ending it.
All while keeping his former teacher (I don't know a better word here...) alive to witness the destruction of the world so he would have his revenge on him.
edit: Since I don't think he is well known, he is a character in the dragonlance universe.
Read Dragonlance Chronicles and Dragonlance Legends.