Well the issue regarding "pain" when one experiences a break up is due to thinking that your love is something special. Fact of the matter is that love is really not special no matter what you consider it to be. Millions of people experience love of some kind, entitlements leads to the idea that your love is somehow special. Which then drives to pain when it's fragile existence ceases.
People can kill you and try to justify it.
That's why I say that my cat is the only creature who truly loves me. If my cat killed me it wouldn't bother justifying it. It'd just eat me. Unconditional love requires unconditional hatred as well. To be truly loved you need to be able to know that the other one can kill you.
And by scary I mean that most people's true selves are quite sadistic or insane.
Not to say humans are not naturally nice. Again that makes biological sense, you scratch my back I'll scratch yours. Monkeys do it, so do humans, dolphins, etc.
However in a natural world that would be the case. Sadly we live in a human world and that changes the playing field.
Most people hide the fact that they have thoughts of killing others. This is due to rules and laws being in the way.
In a natural world humans would kill one another at the very least due to alpha-male fights. Or perhaps due to tribe differences if you put a tiny bit of civilization which we had in a natural world.
People deny themselves that talking to oneself isn't normal, wanting to kill someone isn't normal, and that anyone who does both is just the Antichrist.
Good thing I'm not religious though.