Darth Vader never fully embraced the dark side. He was conflicted. More than anyone, he hated himself. But he was also able to love. Love and hate: two sides of the same coin as some scientists tell us, battling each other within a person who couldn't bring balance to his psyche in his years as Darth Vader.
True evil sith lords relish in the dark side. We call those people psychopaths in real life. They are often empty inside, aspiring only to satisfy their boredom. You would never see them hate themselves. At best, they could get angry with themselves for getting caught. Can you really hate when you've never known what love is? Only raw, empty, primitive anger. Palpatine never hated anything. But being the psychopath he was, he was able to detect the hate in people who could feel ( and love ) and use it for his own benefit.
Darth Vader was neither bored nor empty. He had a burning fire inside him and people, life, feelings, power interested him very much.
He brought balance to the force by destroying both corrupted Jedi and sith alike. In real life, only people who have faced both their light and dark sides can bring balance to society by dealing with pathologies from all sides, even if it takes them long enough to achieve that.
According to the psychoanalysist, Adler, the " regretful" person is often the strongest one. The one who dealt with a great dark in their lives but at later point chose to live in the light, aware of their darkness but now * in control of it*. These people can then guide the rest and help them where other people failed due to their partiality towards good/ evil.