Stella, I think there are a myriad of ways to look at the question you asked. Philosophical, moral, religious, existential, or scientific even. Personally, I believe the philosophical reasoning encompasses most of the avenues so I’ll respond from that direction.
Science would point to the evolution and survival of the species as neurological conditioning in which we learn to advance the species as opposed to annihilation. So a child who slaps or bites their parents (like I described) learns basic morality to keep the species continuing in a productive manner. As we grow older, we remember these principles and act on them, thus keeping the species moving. However, there’s a massive problem within this line of thought.
It’s impossible to leave your own personal moral beliefs at the door because they’re impossible to prove. Let me show you what I mean and dive deeper into your question. Let’s say this:
Person A says we should remove safety nets for poor, get rid of welfare, and let them starve as scientifically it’s survival of the fittest. The stronger species will always survive.
Person B says the poor have right to live. They’re human beings like us.
Person A retorts that scientists today believe the concept of human is artificial and impossible to define. They can point to a greater ethic stating that in order for the majority to live, others must die. Its how nature works.
Person B could then try a pragmatic argument about how helping the poor makes society work better and gives us value. Person A, however, could come up with just as many reason to let the poor die to create a more efficient environment.
At this point most of us would agree with Person B and say starving the poor is unethical! But it’s just as easy for Persona A or anyone to point out that “Who says ethics has to be the same for everyone?”
So then… where do we—indeed—hold these values that keep us from rape, murder, and acting out on our depraved thoughts? Somehow, deep within, we know that even the most mentally retarded man or woman holds more intrinsic value than the award winning horse, Secretariat. Yet we can’t prove that. But there’s still all this darkness within, that would say “Kill the weakest. Punish and torture those that hurt us,” but we don’t act on those thoughts. However, as you pointed out, what happens when we do? I think more recently and in historical events we’ve seen what happens… and it’s not pretty, let alone anything we all want.
For me, the scientific response is weak. Philosophically, it causes us to question the very nature of the human existence. From there, we must ask the question, “Why are we here? And if so, why does it matter?” Because otherwise, if we are giant cosmic mistakes, then why do things like rape and murder matter? Why not given to the darkness if nothing we do ultimately matters?
Quite the question to ponder isn’t it?