I just realized something (after reading through the story again); why didn't he beat and eat all three of the members back then? Nike implied that Alpha was away, so there wasn't really anything to stop him from devouring them; not even the transformation thing, as he doesn't change anymore.
In the Adaptation chapter, Saturno realizes that he doesn't have the desire as he had before to kill Nike, Vinchi, and Venus. He doesn't understand what it means to care for another, but in a sense he has bonded with Nike, Vinchi, and Venus enough that he doesn't want to kill them. He explains to himself in that chapter that he just needs to kill Alpha first then the others. If he did kill any of the others (even while Alpha is away) , Alpha would find out and hunt him down for breaking her rules of the club.
At the same time, there's no way of knowing that Saturno could have beaten Nike and Venus. Nike was wounded, but he is still the 2nd strongest in the group. Venus is stronger than Vinchi and Vinchi is the only member of the group we know Saturno can beat so far. I feel that Nike was wounded because he didn't take Saturno seriously and as a result, was taken off his guard.
Notice how when Venus gave Saturno all of that food out of anger earlier in the comic and he proceeded to eat it all, they were mentioning how their last Omega was a failure. I'm almost certain that the last Omega was the man-eater. The man-eater could speak and hear human words. That, and he called them humans as well which is something only really known by the people in this club. At least, as far as we know!