For those of you who know FNAC lore, Rat, despite the visual similarities, doesn't kill kids. He is no William Afton. He hunts 17 year old Marilyn operating on the rules of TSE - she probably appears to be an adult superficially,
In the climax of FNAC2, the Rat gets his hand free from the puppeteer controling him, the opening of FNAC3 reveals that Mary's brother wasn't actually seriously hurt.
We know that the Rat is capable of MAULING PEOPLE so that people compare it to a bear attack. Yet nothing bad came of Mary's brother despite being near the stage.
The whole idea of Rat trying to get "revenge" on Mary by targeting her family members is purely fanon, there's no indication he blames Mary for what happened-he attacks her the same as he would any other guard.
FNAC3 shows that Rat, in life, genuinely LOVED entertaining kids. And at the end of FNAC3, Mary's brother runs towards the Rat with a big smile on his face, so Rat, who was possessed at that point, probably was trying to reciprocate the gesture, give a high five or pat him on the head as he might've in life.
Because if Rat really meant to do harm, why didn't he do worse?
