As soon as someone's around to witness their crimes they become almost perfectly docile. Arseholes.
It's a painful realisation that they choose to hurt someone, probably because for the most part if we don't like someone we choose to not further engage with that person instead of actively causing harm (exceptions exist of course).
I'm sorry your mother sucks. I hope your partner understands the intricacies of her actions and won't dismiss anything you say.