"Only a Sith deals in absolutes" a hypocritical Jedi-joke which reinforces their belief of the Sith's evil or a sly hint from the conniving Obi-Wan?
Perhaps Obi-Wan was a Sith who believed strongly in the "might makes right" thing, and, to that end, sought that the most powerful of Sith rule the galaxy.
Anakin: What better way to have Anakin fall to the darkside than have Qui-Gon get killed at the hands of Darth Maul? Then denying Anakin any revenge through killing Maul himself. When he discovered his failure, since Anakin was incapable of ruling the galaxy by Obi-Wan's standards, he goaded Anakin into attacking him through his manipulation of Padme. Hoping to finish Anakin then and there, Palpatine's rescue threw a wrench into his plan.
"From a certain point of view speeches": Luke was powerful. Once Luke discovers his lie, anger and frustration at his teacher's deception will surely arise.
His Defeat: He will become more powerful than Vader can ever imagine? Through his leadership of Luke against Vader, and Luke's rage at seeing his master struck down, Luke will fall to the darkside. Letting himself die may seem to point to the fact that he was not a Sith, seeing as how it goes against the tenets of Sithdom. However, Obi-Wan strongly believed that the strong should rule, and to that end, played the Sith for fools.
When Obi-Wan's plan failed, his force ghost period was a constant argument with Yoda, who had discovered Obi-Wan's plan. When Luke saved the galaxy, the final scene was Yoda's way of rubbing it in.