This is not a duplicate of this: Why did Obi-Wan leave Vader on Mustafar?.
The question I mentioned answers why Obi-Wan didn't kill Vader to make sure he's no longer a threat?.
The answer to that question is
Anakin was Obi-Wan's best friend, student and a war buddy who saved his life many times. NOT someone you would be terribly interested in offing.
After cutting down his limbs, clearly one of these two would eventually happen:
1) Vader somehow survives and continues to be a dangerous Sith Lord
2) Vader would die a slow, horribly painful death, basically getting burned alive.
In light of no. 2, wouldn't it make more sense for Obi-Wan to do a favour for his old friend and finish him off, thus sparing him of so much pain and horror that would have been his last moments if he had died on the lava?
Clearly one cannot say "it is not the Jedi way" as from that perspective it makes even more sense to kill him right away. And besides, it's not like Jedi should not kill the Sith they fight with.
How do we explain why Obi-Wan left his "best friend, student and war buddy" to die in great pain?