Well, now I guess we know where the tombstones with the names of Emma, Snow, Robin Hood and Regina came from. So, it seems like something probably forces Hook’s hand, or else Hades gets someone else to do his dirty work for him. But that again raises the question of why Hades needs anyone to do his dirty work for him.
I think having Hook do this is Hades’ way of torture and punishment. Hades is probably fully capable of doing this on his own, but it’s more fun and sadistic to force Hook into signing the death warrants for Emma and her friends and family.