Despite being a big dumb brute, Hillock is someone who can be reasoned with and manipulated to a degree, unlike say Brutus who seems to be way more brawn than brain after Shavronne's second set of modifications. Hillock has the useful traits of being a brute while being just smart enough to follow orders without constant supervision but stupid enough to not question them.

Dominus, while on a mad quest for power, is still a man of 'god' and probably harder to manipulate and more importantly control. Gravicius, on the other hand, is probably more about ambition and less about a moral code, despite his claim that 'he fought for god'. Someone who, for the promise of power and immortality, will follow orders.