1. Leland said that he was a "self-claimed king", not a god. Leland calls him "both god and the devil" as part of a metaphor (which will be explained later on in this review). Gallo's true views of God is that God is dead, not that he is one.
2. Now lets looks at some screenshots from the scene.
Earlier in the film a person who had Pandorum on another ship suffered severe paranoia believe things like a ship being cursed and launched himself and the crew into space thinking it would save them (pretty crazy). Here Gallo is depicted with lightning bolts around his head (which says that there is something going on in there) while having Epistaxis (Nosebleed), obviously depicting Pandorum. We see the other passengers in this state as well, while grabbing their heads in pain, who are referred to as those who had "behaved", suggesting that they were obedient towards him.
Leland said that Gallo casted them out to fight amongst themselves and feed on their own. By the end of the film when Pandorum starts to catch up to to the main character (Bower) Gallo tries to convince him to become a cannibal by telling him that "life eats life". Gallo influence turned the passengers into cannibals, with the enzyme just making them evolve to adjust to the conditions of the ship. They were not trapped in the cargo hold as some have believed, as to be exiled doesn't necessarily mean to be against someone's will.
3. If the passengers had turned into the hunter creatures then Leland should have turned into one of them as well since he too fed on his own (and the hunters) for years. Bower even questions that notion by asking why they hasn't change at all and the response is "maybe they been out longer than we have, longer than even him (Leland)". The ezyme was said to speed up evolution, which by definition is "change in gene pool of a population from generation to generation caused by
such processes as mutation, natural selection, and genetic drift" The trip to Tanis was said to take 123 years and Gallo had committed his crimes less than 8 years after the ship took off. And its revealed at the end of the film they left Earth 923 years agos. Meaning that the hunters are desendants of the crew who have adapt to hunting, fighting, and lack of sun light (which is why their skin is pale. This with explain.).
Some people have claimed plot holes with the backstory but I already explained that. It's talking about the other plot holes.
1."How can the hunters survive hundreds of years without food?"
Well the ship was said to be build to last generations and there was food seen in Nadia and Leland's hideouts, plus there was a deleted scene with Bower eating some food after he leaves his hypersleep pod. I guess food doesn't expiration in the future but it's scifi so whatever.
2."How did Leland knew what happen?"
According to him he heard that from other passengers who had awaken who I guess heard it from Gallo. When he tells his story he says "see that's what some would say".
3. "Why is Bower's non-lethal energy rifle able to blow the head off a hunter?"
While that thing was stated to do damage up close in the film right after he got it.
4."How did Bower knew Gallo wasn't his lieutenant?"
When he gained back some of his memory he remembered what his lieutenant looked like. When he confronts Gallo he says "It seems my minor memory lapse has worn off. Who the Hell are you?"