@matrix "the high ground" was a threat to Anakin. Kenobi was just saying that to him to make him stop fighting.

In Lucas "rhyming poetry" logic Kenobi had the metaphorical high ground. Kenobi had done nothing to wrong Anakin personally. Anakin had let his emotions get the better of him and made too many mistakes. Anakin had made himself lower than Kenobi. The lava is a metaphor that Anakin is literally in hell. He's actually standing on a platform over lava before the "high ground" quote.