You find yourself alone on an island. A large island in the middle of nowhere.
By day, you scavenge for food and avoiding predators, by night you climb up a tree and hope there aren’t any pythons or leopards. Rinse and repeat until you get old and die.
You find yourself on an island with a partner.
Now, even though you have an extra mouth to feed, you’ve also formed the beginning of society. Now you have someone to watch your back. If you’re smart about it, you could sleep in shifts so at least one person is always on alert.
Through discussion, you might discover new ways of surviving the island that you’ve never thought of on your own.
A third person is now part of your band of survivors.
Maybe that person is good at fighting. Now, you can begin to try to build a fortification, maybe wall up a cave while the fighter takes care of any beast that tries to harm you while you’re building your new home.
Flash forward, you’ve got a dozen people. There’s the fish guy, the fruit guy, the health guy, the builder guy, the cook, etc. Your settlement is going well. Even though there are more mouths to feed, being able to focus on a specific task has allowed people to be excellent in their own field of study. You’re better off than you were alone.
Now you’ve got millions of people. You think it’s a bit overcrowded, but then again, everyone is really gaining skill at their respective fields. Plus, with multiple people in one post, there’s redundancy and when one dies, it doesn’t mean the end of the world or the loss of his function.
Life is even better.
But sometimes, one person may want to hurt another. You can’t have that, because if the cook gets hurt then you’ll never be able to taste the delicious Chicken McNuggets he makes. So you stop the person from hurting the cook, and you call it morality.
Morality is a social construct. It’s nothing more than self-interest designed to sustain a society that in turn sustains you.
Now imagine that this process went on millions of years. Every time someone tried to hurt someone else, you tried to stop it. You get used to protecting your self-interest, and dislike violence out of habit. Some might even call it… an instinct.
When you smell blood and especially human blood, you automatically recognize a signal and bookmark it as “there’s been violence”. It aggravates you, it annoys you, it… repulses you. Naturally.
Its become a natural instinct that you pass on to your kids, and they pass on to their kids.
As social animals, we are evolutionarily wired to have a moral tendency. As intelligent people, we know that hurting people is wrong, because harming people harms society, harms our feelings, and in turn harms us.
Next time an apologist smugly asks you what reason you have to be moral without the threat of eternal hellfire, be equally smug and reply: Self-interest – and only an idiot wouldn’t see this.