In the climactic battle of ROTS, where Anakin and Obi-Wan fought, Obi said, "Only a Sith deals in absolutes." But isn't that very statement an absolute? Plo Koon, another famed Jedi Master and general, was also known to see things in black and white, which is absolute. When Obi-Wan said that statement, was that a way of showing that the Jedi weren't very different from the Sith? Or was that unintentional on George Lucas' part?
When Anakin and Obi-Wan were fighting on Mustafar this was the dialogue.
Anakin Skywalker: If you're not with me, then you're my enemy.
Obi-Wan Kenobi: [realising that Anakin is consumed by evil and there's no reasoning with him anymore] Only a Sith deals in absolutes. (Source)
Absolute is defined in the dictionary as being "a value or principle that is regarded as universally valid or that may be viewed without relation to other things." or "that which exists without being dependent on anything else." Normal English translation being - black and white idea that does not take into consideration other events or ideas.
When Obi-Wan says
"Only Sith deal in absolutes"
He means that Anakin's black and white ideology is wrong and that only the Sith would hate everyone who is not their friend.