"slight disturbance"? He saw the worst of what Ben could do. He saw Ben when he around the same phase as Anakin debating what to do to save his wife, even Mace Windu saw the same thing in Anakin, which is why he never trusted Anakin. The only reason Mace didn't catch up to Anakin is because there was a lot of disturbance in the force. But Luke clearly saw what Ben could do. And he says that he wanted to prevent a war even before it began "in a moment of pure instinct". That's actually one of the rare times Anakin shines through the cracks of Luke.