For a pair of supposedly enlightened and un-attached people, Obi-Wan and Yoda sure are obsessed with killing Darth Vader. Beginning with their first conversation in his home, Obi-Wan tries to turn Luke into a Vader-killing weapon. He fills Luke’s head with lies and half-truths and deliberately gets himself killed in front of Luke in order to make Luke want to kill Vader. Yoda not only continues to withhold some key facts (like how Vader is Luke’s father), he also claims Luke can not be a Jedi without first confronting Vader. Since when is taking on a Sith or killing your own father a prerequisite for becoming a Jedi? It’s not; it’s just Yoda’s attempt at emotional blackmail.
The crazy thing is that Darth Vader isn’t even the problem. Darth Sidious arranged the Clone Wars, he arranged Order 66, and he’s the one ruling the galaxy. Vader, meanwhile, is basically just his trained attack dog. So why is Vader the one who absolutely has to die?
For Obi-Wan it’s guilt and love. He loved Anakin (or at least the idea of Anakin) and he needs to believe that man is dead because otherwise he gravely injured his brother and condemned to a life of suffering and slavery. Acknowledging that Padmé was right, that there was still good in Anakin, would mean acknowledging that he, Obi-Wan, could have saved Anakin and didn’t. Obi-Wan needs Luke to prove that he was right, that Vader is irredeemable, and he needs him to ‘fix’ the mistake he made by not killing him outright.
For Yoda, it’s vengeance, except that he’s nowhere self-aware enough to acknowledge that. The Jedi Order was Yoda’s entire life. It defined him and gave him meaning. As a long-lived being, loving individuals was too painful, but he could love and be attached to the Order because the Order was eternal. And then it wasn’t. The boy who was supposed to be the Order’s tool, the Order’s Chosen One, sided with their enemy and utterly destroyed them. Vader betrayed him and and obliterated Yoda’s life’s work. And Yoda hadn’t even wanted him! Everything that went wrong began after they took Anakin in against Yoda’s better judgement. In Yoda’s mind, Vader is the living embodiment of everything wrong in the galaxy. No wonder he has to be destroyed.