100% agree
But he wants to get back to his own world. He won't pass the test if he does that and will be stuck in this world until he 1. Kills the Demon King or 2. Dies and has to restart again. So siding with the Demon King is not an option.
Or actually idk if it restarts when he dies (can he be revived?) but that won't help him get back to his own world at least.
ah yes I forgot he actually wants to return
If he sides with the demon king he won't have the chance to restart reason being because he didn't kill the demon king. He can only be graded than restart after killing the demon king, so if he joins the demon and knowing his op powers and strategic mind he'll most likely die of old age at this point giving up going to his world (づ￣ ³￣)づ
just abandon them lol. side with the demon king together with the elf king