In life, you'll meet people who just rub you the wrong way. This is normal; living isn't an ideal paradise where everyone will automagically get along with one another as one great, harmonous family. People carry an innate "spin" laying in the essence of their being, which like subatomic quarks, fundamentally affect how they'll react to others. This is why you will naturally jive with one person, and really connect with them as if you are both swept along the same current, while with another person, a deep part of you senses that they are a drag, an irritant.
And while you may hear about the horrific things that others have done to each other throughout history, none may prepare you for the personal experience of meeting a person you'd call your worst enemy - someone who you may see others celebrate, but they can, and have deeply hurt you to your soul, without any remorse or consequence.
Often, these enemies are people who have managed to end up in your sphere, and even get involved with the things you cherish. Innately, you sense a deep-seated chasm with that person, but for one reason or another, you neglect to distance yourself from them, and the result? It's like an untreated skin rash, that ends up bleeding out from you scratching it too much, turning into some turgid, festering sore that requires amputation. Suddenly, you may end up in a spot where you begin to lose hope in people generally, and moreso, feel hate seep into your being. Your heart begins to die, and with it, life too. It can be a difficult pit to escape.
The worst monsters aren't those cosmic horror tentacled gods, who in all their fantastic grotesquery look nothing like you. The worst monsters are the ones who resemble you.. except for a single, essential detail, and I don't mean just in terms of physical appearance. It feels disturbing when you're able to recognize parts of yourself in other people who you'd otherwise despise, which tends to be why people often emphasise contrasts with their enemies, as opposed to similarities. "I am NOT like that person.. even though I begrudgingly admit, in another life, in another time, I could have been just as well like them!"
In other words, it's an insight into what you don't want to have.