The "Mother" personality killed Mary because "she" was jealous of Norman feeling affection for another woman.
ðə "mˈʌðɚ" pˌɜːsənˈælᵻɾi kˈɪld mˈɛɹi bɪkˈʌz "ʃiː" wʌz dʒˈɛləs ʌv nˈoːɹmən fˈiːlɪŋ ɐfˈɛkʃən fɔːɹ ɐnˈʌðɚ wˈʊmən.