Were There Any Sith Lords Who Used Their Powers For Good?

Yes, sort of. Two Sith Lords from the Rule of Two era get called decent people. Darth Gravid turned toward the light and tried to remake the Sith in the Jedi's image, destroying most of their accumulated lore before his own apprentice cut him down. Darth Vectivus never dropped the moral code he had as a businessman, refused to chase conquest, and lived out his days at home. Both are Legends. (Darth Plagueis; Legacy of the Force: Betrayal; The Complete Star Wars Encyclopedia)

Darth Gravid burned five centuries of Sith knowledge#

Around 533 BBY, the Sith Lord Darth Gravid was drawn toward the light side of the Force. Not in a moment of weakness, but as a conclusion he reasoned his way into. He came to believe that absolute commitment to the dark side would be the order's undoing, and he began working Jedi values like altruism and empathy into his teaching. (Darth Plagueis)

His apprentice, Darth Gean, did not share the new beliefs. She was appalled by what she saw as blasphemy. (Darth Plagueis)

Gravid did not stop at talk. He began systematically destroying nearly five hundred years of accumulated Sith lore held in his fortress. Gean eventually killed him, but not before he had destroyed most of the artefacts and writings there. A large body of ancient dark side techniques went with them, essence transfer among them. (Darth Plagueis)

That destruction is the reason Gravid exists at all as a character. James Luceno created him for the 2012 novel Darth Plagueis to explain why Darth Plagueis himself, centuries later, did not have access to some of the older Sith teachings and Force techniques. A single Sith Lord's crisis of conscience left a permanent hole in the order's library. (Darth Plagueis)

The story does not have a clean moral. Gravid is one of the recorded cases where alignment with the light did real harm: his conversion eventually drove him insane. Whether burning the archives counts as heroism or as a breakdown depends on how you look at it. (Darth Plagueis)

Gravid also punched a hole in one of the Rule of Two's founding assumptions. The doctrine never seriously considered that one of the two Sith might convert to the light and kill the other. Gravid proved it could happen. (Darth Plagueis)

Darth Vectivus shut the mine down for his workers#

The other candidate is stranger, because he never had a crisis at all. He was just a decent man, by Sith standards, who happened to become a Sith Lord. (The Complete Star Wars Encyclopedia)

Before any of that, he was a human male who lived a normal adult life with a family and a wide circle of friends he cared about. He carried the skills he learned from them into his job as administrator of the Jonex Mine Eight Eleven B asteroid mining colony in the MZX32905 system, near the planet Bimmiel. He set up headquarters on an asteroid called the Home, which was rich in the same ores his company was pulling out. (The Complete Star Wars Encyclopedia; Legacy of the Force: Betrayal)

The problem was underneath it. The colony sat on a reservoir of dark side energy, and it started negatively influencing his employees. A shrewd and principled businessman, he shut the operation down, though he also quietly hushed up the reports of strange happenings to keep it secret. (Legacy of the Force: Betrayal)

Because he was Force-sensitive himself, he then began studying and experimenting with the anomaly personally, and developed a Force technique nobody else had: the creation of dark side empowered phantoms. When the asteroid belt eventually stopped being profitable, he carefully mismanaged the mine into obscurity so the habitat would be left alone and forgotten, then went out into the galaxy looking for the Sith. He found them, apprenticed himself, and rose to become Darth Vectivus, Dark Lord of the Sith. (Legacy of the Force: Betrayal)

The Sith who never wanted anything#

What makes Vectivus unusual is what he did not do with the title. His self-discipline and existing code of ethics stopped him falling for the lure of power that ruined so many of his predecessors. Violence, galactic ambition and the eradication of the Jedi were of no consequence to him. He wanted to immerse himself in Sith lore and study the galaxy, and that was it. (The Complete Star Wars Encyclopedia)

Years later he returned to the mine as a fully fledged Sith Lord, built a mansion in the heart of the asteroid's dark side potency, and made a personal Sith holocron. Its contents were his philosophy on business practices and a detailed explanation of how to raise the phantoms he invented. (The Complete Star Wars Encyclopedia; Legacy of the Force: Betrayal)

He lived out the rest of his days in comfort at the Home, surrounded by the people he loved, and his spirit lingered in the mines beneath it long after his death. For a Sith Lord that ending is close to unheard of. He was not soft, though. The businessman in him stayed ruthless, willing to do whatever it took to succeed, and he encouraged the same in others. He just had a floor he would not go below. (The Complete Star Wars Encyclopedia; Legacy of the Force: Betrayal)

The Vectivus problem#

Vectivus's real damage came long after his death, and he did none of it himself. (Legacy of the Force: Betrayal)

By 40 ABY the Dark Lady Lumiya believed the galaxy was collapsing into anarchy and that a Sith with Vectivus's moral stature was what it needed to restore order. She used him as the argument. Posing as Brisha Syo, she lured Jacen Solo, his cousin Ben Skywalker and the Jedi Knight Nelani Dinn to the Home, separated Solo from the others, and walked him through Vectivus's life as proof that you could take dark side power without the evil that usually comes with it. (Legacy of the Force: Betrayal)

It worked. Solo was eventually convinced by the example of Vectivus's solid moral foundation. He became Darth Caedus. (Legacy of the Force: Betrayal; Legacy of the Force: Sacrifice)

So the one Sith Lord who genuinely used the dark side without becoming a monster ended up as the recruitment pitch that created one of the worst monsters of his era. Even the One Sith, who regarded Vectivus with scorn, respected his phantom technique enough to destroy his habitat rather than risk the Jedi learning it. (Legacy of the Force: Inferno; Legacy of the Force: Fury)

Both these stories are Legends and neither has surfaced in Disney canon. Which is a shame, because between them they make a better case against the Sith than most of the villains do. (Darth Plagueis; Legacy of the Force: Betrayal)