In the novel, just image the kids Shogo had to kill... and he still kept his sanity.
The realization that The Program has been around for years, meaning that hundreds of children have gone through the same thing.
Thousands, even. In the novel, at least, it's been going on for 50 years, and 47-50 classes of ~40 students are selected each year - something on the order of 100,000 students have been put through the grinder.
94,000 at minimum, that means 2,350 people have survived the game, and how many of them were already budding psychos because of how the most ruthless or crazy the winners would mostly be?
In the novel, Sakamochi tells his superior his wife is expecting their third child, as they are eager to fight the dwindling youth in the country. Pause. Think. Despite being the cause of said dwindling youth, the country wants more kids. They are encouraging people to have kids that may end up in The Program.