It’s entirely possible that Snow White became Mary Margaret without knowing the origin of Regina’s hatred. Regina defiantely went from Sister, Sister to Mean Girl after the events we saw and being secrative isn’t out of teh question. She could also have wanted to destroy Snow’s chances at Hapilly Ever After just as Snow initiated the death of hers.
There’s also more of a potential for drama by not telling her the truth. When Mary Margaret does regain her Snow White memories (and let’s face it, that’s the direction we’re heading in, possibly and probably by the end of the season), she can confront Regina.
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