Only a Sith deals in absolutes
I think the scene is very evil when understood in the modern context (as Lucas and non-objectivist viewers would understand it).
Anakin: "Either you are with me or you are against me"
He names the facts - there is no possibility of them going their own ways peacefully (he has an order from Palpatine to hunt and kill all Jedis, and Obi-Wan has an order from Yoda to hunt and kill Anakin, not to convert him back to the light side).
Obi-Wan: "Only a Sith deals in absolutes"
Obi-Wan does not answer him at all and does not try to reason with him, what he says is not a true or false or even self-contradictory idea. He needs to give himself courage to kill his beloved disciple. He needs the strongest damnation of Anakin so that he can do it. In this context, he doesn't choose to say "You are a traitor", "You are a child-killer", or even "You are selfish". The strongest proof of evil is not any of those but "Only a Sith deals in absolutes". His damnation of Anakin is not based on his actions or character, but on the way his mind supposedly works. And he succeeds - it gives him the needed courage and determination - he switches on his lightsaber and fights Anakin.
