The beauty of it all is he's not sensible enough to create a barrier so that the speck of dust/smoke from the fire won't be ingested by any living being and corrupt them.
He's cautious, sure, but he's stupid through and through.
@DragonTiger: No..wha..how...whaaat? 😁 Is that just a theory or a spoiler? (BTW: If it's a spoiler don't forget to put it in [spoiler][/spoiler]-tags.)
What a crappy partner to have in that goddess. The same trait she is mad at him for is the same one that saved her ass. If he wasnt like that they would have came to the world at level 1 and he would have instantly died once this demon king showed up.
Also she doesnt want him to accept any reward money for his help yet she cant help him get any material in that world. So how is he suppose to live? She really doesnt think anything through.