One of my friends is the "fight-chief" (what we might call "warlord") for a small language group in ... another country. He led raids where he and his men would wipe out entire villages, surrounding them, then coming in burning every house down to the ground and killing everyone in their path. This man had tasted human flesh. When I first met him, he was the human embodiment of evil. His expression told you that he would love nothing more than to bite your head off, literally.
One day, shortly after I met him, he was reading the newly translated account of Cain and Abel. After Cain kills Abel, God comes to Cain and says to him "the blood of your brother is crying out to Me from the ground." My friend told me that when he read those words, he felt like a spear went into his side, intense literal physical pain so that he could hardly walk. He said he felt that God was saying to him that the blood of all the people he had killed was calling to Him from the ground. Shortly after, he repented and made an astounding turn around. Over the coming weeks, he instituted a peace pact with their main enemies and held to it in spite of their breaking it several times. The last break came when five enemy clans banded together and attacked his village, burning every house down to the ground. By a miracle, none of my friend's people were killed, but they lost everything. Contrary to cultural expectations, he refused to retaliate and instead continued to speak to his people and his enemies about peace and reconciliation. That attack was the last of a war that had raged for several decades. For the last 8 years, there has been no more fighting, and I am now looking at a picture of this man holding my youngest when she was three years old. The expression on his face is so entirely different from when I first met him - his eyes are shining and are no longer dead like before; his smile looks honest and warm, rather than like the snarl he used to wear.
The point is, no man is beyond repentance. The further a person sinks into evil, the harder it is for them to come out, but it is never impossible. And I think Darth Vader's story illustrates that. The way I interpret the movie is that the Jedis thought it was impossible, once having gone to the dark side, to ever come back. Yoda said that once you go to the dark side, "Forever will it dominate your destiny". But Yoda and the Jedis were wrong about that. Luke proves it by being the instrument through whom Anakin is redeemed.
George Lucas may or may not have intended for the Jedis to be wrong in thinking that the Dark Side was inescapable. But no author can ever craft a perfect character, and truth has a tendency to come out. In this case, the truth that anyone can be turned comes out in spite of some of the characters having been written to think that it was impossible.