Obi-wan's last words to Darth Vader:
"You can't win, Darth. If you strike me down, I shall become more powerful than you can possibly imagine."

Obi-wan's last words to Anakin Skywalker:
"Goodbye old friend. May the Force be with you."

What happened? Things sounded like they were going so well.

It's a beautiful sunny day on Coruscant. Anakin and Obi-Wan are standing on a loading platform, Obi-Wan about to head off on a crucial mission in the Clone Wars, the confrontation of General Grievous. Anakin and Obi have a nice chat. Compliments are passed, Anakin even atones for some of his mistakes. Obi-wan says goodbye.
Then Obi-wan goes off to Utapau to win the war, a confrontation is forced between Mace Windu, Palpatine, and Anakin, and lo and behold, Anakin falls to the Dark Side.

Their next meeting would be on the hellish Mustafar. Anakin is busy choking his significant other, and suddenly master and apprentice are on polar opposite ends of the spectrum.

A lot can happen in a few days.

They argue, they fight, Anakin burns, Obi-Wan laments, and they say their final goodbyes:
"I HATE you!!
"You were my brother Anakin. I loved you."
(Note the past tense) Sadness grips his voice, and he painfully leaves his evil friend to die. He is crushed, and Anakin isn't doing too well himself.


But a lot can happen in 30 years. Emotions cool off, flesh cools off, and Anakin steps fully into the mantle of a Dark Lord. Obi-wan, even in his isolation, hears of the evil deeds perpetrated by Darth Vader. The hurt was so bad, and the deeds were so great, that Obi-wan completely disassociates Anakin Skywalker and Darth Vader. He convinces himself, as he convinces Luke, that they are different beings, and whoever Darth Vader used to be, Anakin is dead, and dead forever.

Their final confrontation is cold and brief.

Obi-wan isn't done talking yet, though, and he continues to give guidance to Luke. A few years later, Anakin Skywalker returns, and moves on into the Force, where he and his old master can finally once again stand as friends, and perhaps share a few more last words.