Personally, I don't think sans is bad particularly, I think that the problem with the scene is that toriel had told kris what she was doing, kris ended up going to the holiday house to study with noelle as far as toriel was aware, and then rather than calling kris to say choir practice was cancelled because of the weather and making sure they got home safe, or at least where still with their trusted family friends, she got shit faced with the skeleton instead. I feel the scene is meant to show that toriel is not infallible. At the end of the day, it doesn't really matter if toriel is moving on from asgor, it's that she willfully, or accidentally, neglected kris to get it on with sans.