Obi-Wan Kenobi: "You have allowed this dark lord to twist your mind, until now, until now you've become the very thing you swore to destroy." Anakin Skywalker: "Don't lecture me, Obi-Wan! I see through the lies of the Jedi. I do not fear the dark side as you do. I have brought peace, freedom, justice, and security to my new Empire." Obi-Wan Kenobi: "Your new Empire?" Anakin Skywalker: "Don't make me kill you." Obi-Wan Kenobi: "Anakin, my allegiance is to the Republic, to democracy." Anakin Skywalker: "If you're not with me, then you're my enemy." Obi-Wan Kenobi: "Only a Sith deals in absolutes. I will do what I must." Anakin Skywalker: "You will try."
Obi-Wan: "I have failed you, Anakin. I have failed you." Anakin Skywalker: "I should have known the Jedi were plotting to take over. Chancellor Palpatine has showed me the true ways of the Force." Obi-Wan: "Anakin, Chancellor Palpatine is evil! The Sith are evil. The Dark Side of the Force is an evil presence." Anakin Skywalker: "From my point of view, it is the Jedi who are evil." Obi-Wan: "Well, then you really are lost!" Anakin Skywalker: "This is the end for you... my former master."
Now that Obi-Wan has seen there's no going back, he must face Darth Vader one-on-one. The two argue over their beliefs and Obi-Wan realizes that Anakin Skywalker is no more. He has to do what he has to do.