@Touyue most of the heroes were people in position of power, or people that were put on positions of power, the "environment" was great, they were wealthy and respected, besides the heroes, Norn and the King, no other character on the manga is evil, they were either following orders or plain good people.
The heroes being evil, that was by choice, the "environment" have nothing to do with it.
The protagonist is punishing people for crimes they did not committed, he had the choice to make them into decent human beings, unite the world and achieve absolute peace, but instead he wants to get revenge, he could change everyone and them change himself to forget about his past life, he have many many choices, but he chose to get revenge for crimes that did not happened yet, to me, the Healer Hero is as bad as the other Heroes.
The world is only toxic to the healer hero and the demon lord, the number of actual evil people is very small and again, its something that he could fix with a snap of a finger now.