When is understable why he wouldn't use his power on a fellow human he should have just told them to find a venue for it, get some bodyguards and test it on them, getting fired because he refuse to do that is retarded.
(I said bodyguards because the only reason to not make the test is if he fears a person getting hurt/killed when debuffed, in some other stories I've read it happened that a skill used as a joke ended up killing someone.)
Even more if how the title implies he gets hired from the enemy and will end up using his magic only on humans, possibly fighting them outraged of having been fired.
If the spoiler about the coming chapters is right the author just made the starting part because he wanted to put a emo MC that felt betrayed by his comrades, when is simply not the case, even his old friend would go for efficency in a "dangerous" expedition.
I just know that MC will leave and the party wil get a sexy healer, and then be unable to pass the next floor and believe the enemies have become ultra strong, they will give the fault of it to the it being the ultimate dungeon and having to get used to the new party member.
Then they will retract one floor thinking they have to get used to each other on a "easy" zone and finally understand how much the debuffs were helping.