Muzan Kibutsuji. And I'm not kidding either, he's the best option here. Red Skull, Joker, and Yhwach will often kill their minions just for the hell of it. Darkseid will kill you once you've completed your task and rarely will lend any credence to the thought that you could still be useful. And sometimes he kills people for no reason too. Diavolo is less likely to kill you for no reason, but he will use brutal tactics to keep you in line even if you are actually good at your job. As seen with Rui and some of the Upper Moons, if you are good at your job, Muzan will actually leave you the hell alone, making him the only person here who if you're useful, you won't be tortured or murdered for no reason.