The Tragedy of Darth Sidious the Wise
Did you ever hear the tragedy of Darth Sidious The Wise? I thought not. It’s not a story the Rebellion would tell you. It’s a Sith legend.
Darth Sidious was a Dark Lord of the Sith, so altruistic and so wise that he eventually turned to the light side of the Force to see the future. He ended the Galactic Civil War, and built a utopian empire to combat the evils of rebellion and war. He had such a knowledge of the light side that he could even keep his beloved apprentice, Darth Vader, from dying at the hands of the Jedi.
The light side of the Force is a pathway to many abilities, which can be used for the good of all. He became so steeped in the light that the only thing he was regretful of was his past, which included allowing Anakin Skywalker to be consumed by the Dark Side.
Unfortunately, his apprentice tried to recruit a young farmboy, Luke Skywalker, to the Sith. The naive Skywalker, brainwashed by the Rebellion, refused to accept Darth Sidious’s generous tutelage of the Light and the Dark in moderation. Fearing that Vader would succeed where he had failed, Sidious chose to mercifully end the boy’s life rather than let him become consumed by the Dark Side as Vader’s apprentice.
But Darth Sidious was too noble, even for this mercy killing. Unable to direct any negative emotion at the innocent Skywalker, his lightning inflicted no damage besides pain and left Skywalker unmaimed, compared to its devastating effects on the traitor and conspirator Mace Windu. In this moment of weakness, Darth Vader betrayed him and threw him down a reactor shaft, where he chose to accept atonement for his sins rather than save himself with the Dark Side.
How ironic - that the most restrained and powerful practitioner of the Dark Side turned to the Light and thus was destroyed by the one thing he feared most - his past, which struck him down in the form of his apprentice.
From https://www.quora.com/What-would-happen-if-Darth-Sidious-turned-to-the-Light-Side