I’m not here to accuse anything, but would still be curious. Could there be some noted hypocrisy going on in his household. Like adult relatives who make a big point of looking good to the world, but are themselves given markedly to some vice in the household. Kids learn as much from example as from moralizing. “Don’t do as I do, do as I say do” is vain for directing a child in life. Not saying that’s the only way this degree of evil can be fomented, but it’s ONE way. Lack of kindness sometimes has roots that pass through earth.
Family appears very normal. I’ve asked all these questions and don’t know what to attribute his evil to. Sometimes crap just happens.