First off, these were children, they were killed in 1987 and their bodies were stuffed into these animatronics who are trying to kill you, their souls are now possessing these animatronics. The nightguard is seemingly acquainted with the place and "those doors" even more so the animatronics claiming to see someone that looks just like him. Presumably the nightguard is related to the murderer, since the murderer presumably is the owner of Fazbears Diner A.K.A William Afton, meaning they want to have revenge against their killer. In short, knowing the story of FNaF, this song would make clear sense, as it gives clue to who the guard is, why they are mostly to kill you.