There was a lot of discussion and debate about Obi-Wan's statement to Anakin about the "high ground." Here are my thoughts about it...

I think Obi-Wan's comment about high ground was related to the geography of sword fighting. However, I think he also meant it as an opportunity for Anakin to feel and demonstrate some humility and even forgiveness.

Just before, Anakin was able to leap over Obi-Wan from one floating platform to the platform Obi-Wan was riding. So he might have been powerful enough to leap over Obi-Wan again. But I think Obi-Wan realized that the height was greater and gave Anakin the chance to feel humility regarding his power rather than arrogance.

And by telling Anakin "don't do it, Anakin" he was trying to get his former apprentice to realize that Obi-Wan didn't truly want to fight Anakin; and that Obi-Wan was trying even then to reconnect their relationship.

When he ignored the opportunities for both humility and reconnection, and indulged his arrogance and anger towards Obi-Wan, by trying to leap over his position, Anakin finally and fully became a Sith. And Obi-Wan ceased to see him as Anakin and at that moment accepted the fact that his "brother" was now a Sith Lord, although in Obi-Wan's mind, a Sith Lord that was about to lose. So Obi-Wan had no qualms about taking Vader's arm and legs, and then leaving him to die.