luke wouldn't have changed his mind about saving his father from the dark side even if he heard what anakin did in the jedi temple during order66, because he literally knows about it. that's the first thing obi-wan tells him about darth vader, is it not? that darth vader helped the empire hunt down and destroy the jedi order
darth vader tortured leia and stood by while alderaan was destroyed, he shot luke's friends over the death star, he used han, leia, and chewie as bait for luke to try to kidnap him, he cut off luke's hand, ect. luke spent several years fighting for the rebellion, so he's heard all the horror stories about darth vader, about all the people he killed or who the empire caused incredible pain to
luke is not an idiot and he is not ignorant. he knows about all the bad things his father did, and he wants to help him anyway. because he can feel him in the force and knows that there is good left in anakin, because he feels the desperation and fear with which his father tries to reach him in the force, because luke understands who vader is to the emperor and why he unquestioningly follows all his orders — luke's mentors told him that his hero's journey wouldn't be complete until he slew the dragon, but luke saw that the dragon was a pathetic, broken, chained man with goodness still alive in him, so he decided to save the dragon instead
the light and dark sides of the force exist in-universe as real concepts with their own rules, but they're also just allegories for the vague concepts of good and evil in your beliefs, words and actions. remember that star wars doesn't tell a story about good people and evil people, it tells a story about people who can choose good or evil. if you think about it, both the jedi and the sith didn't believe in the possibility of returning from the dark side, essentially denying the very concept of redemption, and both were proven wrong
anakin is the perfect character to be redeemed in this specific story because once he was a genuinely good person, who started out on the path of evil for reasons that would be understandable to any human being in this world (fear of loss/change), and stayed there because of the very false belief that the jedi and sith convinced him in, and which this story criticizes ('it's too late for me, son') — luke skywalker knows all of his father's sins, but loves and believes in him despite them, and the narrative rewards him for it
the reason why the narrative rewards luke skywalker for his faith in his father is because george lucas is trying to convey to the audience the idea that it is never too late to choose good, that redemption is unconditional and available to anyone at any time, that redemption is rewarding — darth vader fits this story perfectly because he is so 'twisted and evil', not in spite of it. at no point does this story turn away from the evil darth vader committed, but it says that he still can change for the better, and so he does
it's such a clear simple story that it still amazes me how people can fail to understand it
