But Luke and Vader never on the inside actually WANT to be EVIL. Anakin turns because he hates the Jedi code, believes the sith are the light side and the Jedi are the dark side. Once he is lost in his power and grandeur, gets his legs cut off and becomes more machine than man that he might as well throw himself into his work. Luke is tempted by rage and if he killed Vader he would believe his friends were dead by then, the rebellion was crushed, all hopes of converting his father were gone and he had the emperor waiting for him with open arms to rule together. Luke would choose the emperor at that point. By him not killing Vader he chooses to do the noble thing and die rather than make the previous mistakes of his father. Then Vader cleans it all up himself by killing the emperor and realizing he fullfills his true destiny and that neither the sith or the Jedi of the old republic were right but that Luke had shown him back to the good path.
Now by having somebody motivate you to turn to the dark side by saying “turn to the dark side” it is so weak! Above I explained how intricate manipulation of ones moral compass and friends around him can turn him to the dark side. Not simply repeating in many forms of “turn to the dark side”. I feel like these lines in ROTJ cheapen the turn of Anakin in RoTS and Lukes possible turn in ROTJ.