Not everybody is redeemed the Emperor was beyond redemption, so was Exar Kun, darth Maul and Darth Bane. and a character who dies at the end of the legacy series.Callista never came back from the darkside.

Ulic Qel Droma was redeemed, so was revan. Luke Skywalker came back from the darkside just Like Anakin Skywalker did. Cam Solusar came back, Zekk, Mara Jade. Kyle katarn. Kyp durron. Bastila Shan. Lots more probably.

I think its more shades of grey really even though that is not how star wars was originally conceived. Things are not so clean cut between light and dark. The Force is not constrained into the narrow dogma of the Jedi, despite their being keepers of the peace and mostly Ethical beings. I think Yoda and Kenobi where wrong they thought Vader was beyond redemption. The old ways that bound the jedi like not being able to Love or have a family or children were their undoing. The foundation of the New Jedi order under Luke came about because Luke believed in the good in his father and forged a respect for the man He once was and not the man he became, he reached out to that one last spark of light. That was Anakin Skywalker in Darth Vader. The emporor also thought Vader beyond rememption just like Ben and Yoda.