Okay.
I criticized this story 2 times in this arc because the MC had a chance to kill this bastard twice but didn't.
And it felt back then that it was to give the guy some pain as punishment because killing him would have been too fast and ended his pain.
This ending made MC actions logical and with a thought to the future, more than I'm used to from isekai stories, I must admit the author played it perfectly.
I would have never guessed this guy would have been used as substitute because we didn't know anything about him aside his being bad, the wanting to be a immortal, elfization or tricking the tree were just something we wouldn't have guessed, as at most we thought that stopping the bastard using the tree mana would have been enough to fix the problem (and if MC did kill him early on maybe it would have been enough).