When some poor guy has a mental disorder where he sees everything as an organic mess from the world around him to his own friends, he makes a new companion in the form of angelic girl named Saya... Of course she turns out to be pure evil. I don't remember when exactly this song plays, but its generally when things are going south.
By the way: For the sake of your sanity, never play or watch this game with censoring off! This is something that has truly earned its reputation for its grotesqueness.
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