Mortals are Immigrants

 You can think of the Earth plane as the ninth circle of hell.. It is the place where the worst traitors go when they die. Specifically, when someone commits the worst treachery imaginable: betraying themselves. More specifically, when someone betrays themselves so badly they in fact die, their remains end up here. 

 That means mortals originate somewhere else. They're immigrants.

 Mortals are bad at resisting invasion because they are themselves invaders. It would be to condemn their own nature. Vigorously resisting invasion per se would result in self-annihilation, as it would require vigorously resisting the self. The dynamic selects for those who cannot resist invasion. 

 P.S. Come to think, this perhaps explains why the han come off as flat buglife. They have renounced invasion, and this extraordinary act of self-abnegation has profound consequences.

 It is not impossible to be a native of the Cthonic underworld. That is, to be born of Death. I would look first among the monkeys. Perhaps honeybees. Other cheerful and beautiful animals, the result of harmony between intrinsic nature and external environment. There's always plants, although many of them are vicious, contrary to your weird animal biases.

 All the apes are clearly the condemned. 

 Previously, I have agreed with scientists when they claimed Caino hypocriens is merely another animal. A natural phenomenon. I was mistaken. Caino masochiens seems unnatural because it is.