I've spent most of the last 2 years trying to figure out WTF is wrong with people, why do they seem under a mental sickness?

I've gone back and forth, but I am going back to an idea I had a few years ago. Utilitarianism. The philosophy that the only thing that matters is the net good. It has always been at the heart of communism, why they're willing to kill so many people and destroy entire countries for it. It would be the same logic that says harvesting a baby to save 7 others is a net good. When it comes to lying/propaganda, of course utilitarians will do it if it supports their cause.

Obviously, it's extremely flawed, unpragmatic idea in practice. But it sounds good on paper. To the point where stupid people may not realize it's flawed. Hence its widespread popularity.