I just realised something that seems pretty messed up after re-watching the Obi-wan vs. Anakin fight

When the fight was over and Anakin was burning alive why would Obi-Wan not put him out of him misery? I understand they were enemies at this moment, but Obi-Wan even says that Anakin was his brother and he loved him. So he just left him to burn alive in very obvious pain? Anakin was completely defenseless. A quick little lovetap to the skull with Obi's lightsaber would have saved him so much agony but he just walks away to let him burn. Seems kinda messed up.