One question people regularly think of when I tell them that Satan made this world is “why isn’t it worse?”, meaning, why are there good things in it at all then?
I went through that in the last book, but there are some things that may be added.
First, he didn’t actually get what he wanted. He didn’t actually design the world; he made a wish list of parameters he gave to the Jewel, who did all the heavy lifting. Sanat Kumar was like an idiot boss who has a genius employee. He makes his idiotic requests, and the employee figures out how to get the closest to that with what’s actually possible. There were things about the world Satan didn’t like. Like, at all. He tried to change them, but then the world didn’t work. Like, the matter didn’t fuse on nuclear or chemical level. Life didn’t happen, or it remained single-celled forever. Or life happened and was quickly extinguished. Or the species that he wanted got extinct. Or the souls didn’t bind to the bodies. Basically, this is as bad as he could get it before the whole thing became unliveable.
Also, the Gods had a say, as I wrote in the book. They intervened to a point; its very foundations are cracked, allowing transcendence to manifest despite Sanat Kumar’s intent and commands, because the most fundamental layer of everything exists because of God, and the Jewel exists because of God’s plan and intent. This forced Sanat Kumar to just live with some things he absolutely hated, and wouldn’t have allowed them to exist if it were up to him. So, he intervened on the more superficial layer of civilization, when the layer of world architecture couldn’t be pushed beyond a certain point.
The thing is, Satan was never quite happy with what he got, or how it evolved. Some things he hated always seemed to emerge, no matter how hard he tried to suppress them. He would design the system so that oppressing and abusing others was more profitable and effective, and people would still find a way to love someone and care for them, making him irate. The selfish properties he promoted and tried to make more effective ended up creating profound unhappiness despite his propaganda. He designed everything so people would win by playing according to his rules, and most chose “mediocrity” or even “loss”, completely eschewing his rules and the game itself. Almost nobody wanted to win according to his definition of a victory. Instead, they instinctively approximated behaviours they remembered from Heaven, on some layer that wasn’t memory, so he couldn’t completely suppress it. When people embrace his rules and ideals, the entire structure collapses and they notice.
That doesn’t mean that this place is good, and that God is happy with it. Quite the contrary. It’s terrible, but not as terrible as Satan wanted it. He didn’t get his way completely, because his ideas are so fundamentally bad, that reality collapses when they are implemented fully. This leaves a broken compromise with what Satan wanted, and what he could realistically get, softened additionally by the fact that he was allowed to do this in the first place because God had his plans and ideas that had precedence.
So, that’s why beautiful things exist here; that’s why this place is a mixture. That’s why brahman permeates the world the way butter permeates milk. That’s why baby wolves are cute, and societies function when people instinctively emulate aspects of heaven on every level, and they fall apart when Satan gets his way. He designed it, but everything is made upon God, and within God’s mind.