Originally Sauron was a Maia who was 'good and uncorrupted' with a passion for order. He sided with Morgoth because he thought if they could control the minds of Middle-Earth they'd be better off. As far as I know, in the lore, Sauron isn't evil in the way he's portrayed in the movies in that his goal is to improve everyones lives through efficiency and order (so it's not as simple as light v dark).
Sauron can shape-shift so in the second age he shows himself as a fair and beautiful man, but he's also portrayed as a werewolf and vampire in the books at different times!
Interesting. Didn’t he lose that power when made the final Ring of Power, and when he activated it, he threw all of his “life force” in it (cruelty, malignancy etc.)
Yes in order to make the One Ring more powerful than the other rings he had to put his own strength into it. However the elven rings weren't made by Sauron so he couldn't control those (eventually worn by Gandalf, Galadriel, and Elrond), and if I remember right the Dwarfs were too stubborn to be controlled (or something), so only the humans ended up being controlled anyway.
Sauron can shape-shift so in the second age he shows himself as a fair and beautiful man, but he's also portrayed as a werewolf and vampire in the books at different times!