Also, in ROTJ, Luke has been communing with the force for a year. While it was cut from ROTJ, that includes being in touch with Vader himself. Being in contact like that, even just the short bit we see in TESB, would let Luke past Vader’s guard and let him see the depths. He would sense any conflict. Luke’s entire plan to defeat Vader (and maybe Palpatine in the process) was to pull Vader back to the light side. He utterly failed until he sacrificed himself to Palpatine. We as the audience do not get to see Vader’s conflict, but we do get to see his change of attitude in TESB. He is willing to destroy the Emperor. That isn’t the attitude of a loyal henchman as he was portrayed in ANH. And with the revised Palpatine scene with Ian, Vader outright lies to Palpatine, pretending he doesn’t know Luke is his son already. So there is plenty layered in there to hint that redemption might be possible. Add in the PT and it becomes very clear that Vader is excited to be a father in TESB, but is still trapped in his evil and wants his son to join him. Instead he joins his son. And the only way to believably do that is for Vader to give his life for his son. Leaving Vader alive would result in a trial and execution anyway. Better he has a hero’s death.