That arises from a hunger for power, a desire to control someone else, itself an animal instinct. It's well known that rape among humans is mostly a demonstration of control; physical abuse is also showing control, because you're demonstrating the other person is powerless to stop you. Animals don't burn skin off because they lack the motor ability, tools, and intelligence to do that. Garden-variety physical assault happens, though -- male dolphins will beat their victims before raping them.
Again, that sort of assault is purely functional, both in ducks and dolphins. Beaten victims are less likely to resist. Do you know of any dolphins beating their victims
after raping them? That would be more indicative of what we're talking about.
Also, it's noteworthy that the species most likely to rape or torture for the sake of power(meaning homosexual or inter-species rape) are apes and dolphins, who are both highly social and intelligent. They engage in rape and torture because their minds are closer to human ones, not because they are more animalistic than humans.
Their lust was not solely sexual -- it was about control. The physical abuse was just as important to them, as they kept her even after "they had lost sexual interest in her" (Wikipedia), whereas a previous girl was let go. They did not think about the harm to their "future," because the animal brain making them commit these atrocities has no concept of "future," only the here & now. After they killed her and the reality of what they'd done was able to sink in, then they started worrying about the future.
True. But since we're talking about their previous crimes, take note of the dramatic shift between their previous rapes, functional in their nature, and their MO in case of the last victim. What is the cause of such an increase in brutality?
The one difference that springs up is perceived "slights" against them: first, rejecting a relationship with one of them(which seems to be the difference between them letting their first victims go and keeping the victim in the latest case) and later, trying to call the police(after which they escalated to life-threatening forms of torture). In other words, their main motivation for engaging in such vileness was not the animal thirst for power: it was a human desire for revenge.
Taking revenge after the threat was removed(as opposed to stopping once you have protected your interests) is a concept limited to very few social, intelligent animals: apes and corvidae. Again, that sort of abstract motivation is closer to human behavior than to that of animals.
I agree on the general point: humans are assholes, in part because of the more animalistic emotions like thirst for dominance.
But it appears that humans'
purely emotional, not even physical or intellectual, capacity for evil has far surpassed that of animals, and therefore, blaming the animal brain for human deeds is like blaming Reagan for Bush-era invasion of Iraq and subsequent rise of ISIS. There are definitely some precursors of what happened in case of Reagan, but his policy is not the
main reason for the escalation that happened decades later.