The main point he picked out was this statement
My whole system came about because I wanted true evil, rejection of creation, but I wanted shades of gray in religion. In my reading I know that "evil" religion don't exist in reality.
The problem with evil religions in D&D is that they are not realistic if they are part of a culture or nation. My definition of a evil deity is somebody like Lolth, Elemental Evil, Bane, Howard's Set, etc. They are not realistic because throughout human history cultures just don't go into worshiping evil deities. Isolated cults yes which often manifest as small groups or individuals rejecting the larger culture they are part of.
The technical definition of these type of religions is Maltheism.
Maltheism (from "mal" meaning bad, or illness, and theism, from... well, theism) is the idea that god is just out to get us and that he or she or it is malicious, like a kid who keeps selecting "Monster" from the disaster menu in Sim City. A Maltheist, therefore, is someone who believes that a god or gods exist, and that they are evil, malicious, incompetent, or otherwise causing the suffering of humanity.The Cathars of southern France during the Middle Ages believed Earth was under the sway of an evil god, the Demiurge. The Demiurge was equated with Satan. But being a Cathar wasn't about worshiping the Demiurge. Their whole faith was about rejecting the world (mainly through a severe form of asceticism) so their souls would be put on the road to the realm of light where God exists.
This doesn't mean cultures haven't adopted religions or philosophies that were considered abhorrent by neighboring cultures. Also various cultures adopted religions that essentially turned their nations into totalitarian states with all that entails.
The "evil" religions have some fig leaf that makes it appealing to members of that cultures. Usually by preaching some type of racial or cultural superiority. The worst practices of these religions (and they can be pretty bad) are reserved for those they conquer or capture. But among those in the "in-group" the religion can be benign. The Assyrians around 800 B.C. are a good example of this.
The way I apply this to a fantasy campaign is rather simplistic. Religions involving a truly evil deity have cults and never dominate a nation or a culture. The rest have various fig leafs to make them appealing to the cultures that adopted them.
In my own campaign cultures dominated by Set are totalitarian and tyrannical in nature. The culture worshipping Hamakhis believe that the practice of human sacrifices aids the god in holding back the chaos that will destroy creation. And that culture has an ongoing reformation that is trying to return the worship of Hamakhis back to it's older form in worshiping him as the Judge of the Dead. The last evil god of campaign, Kalis, is a nature goddess of blood and revenge. She is only worshiped by cults.