As I keep saying most men are weak.
Most people are NPCs.
Most people are selfish.
Why?
Because that is the default and the default requires no effort.
In other words because it's easier.
It's easier to be weak than it is to be strong.
It's easier to be an NPC than to be a non conformist independent thinker.
It's easier to be selfish than it is to be considerate of others.
All it takes for evil to flourish is for most people to choose the easy way.