Joan Wasn't A Good Mother
Bernard was not the only self-centered or mean-spirited character in this film; I think Joan had a considerable amount of narcissism too.
"Mommy needs some nights alone"? Despite the joint custody dictating those terms, that seemed a cruel thing to say to a child starved for his mother's company (and who, in normal circumstances, would be a full-time parent). Instead of comfort, she dished out pseudo-bohemian "coldness", as if she was above warmth & protectiveness, to her own children. It was as if indeed she wanted to rid herself of the task of even half-time mothering (and feed her ego with her literary ambitions & other gratifications).
Calling her teenage son a *beep* etc. There were reasons Walt resented her, apart from being influenced by his pompous father. (And, at least, Bernard wanted time with his children, even if he also was egocentric and involved with Lily).
He might have been a bad father, yet she was an equally terrible mother. I understand no-one is perfect, BTW, and that it's a film, but that's my take. Father-bashing has been so common for some time, I think many tend to turn a blind eyes to bad mothering, or make excuses for it, blaming the father for all & holding him accountable for everything.
The two, in this case, were to blame.