I think Dee is an incredibly tragic character tbh. She was the one of the group who had the most potential from the outset. A nepo baby, sure, but not as advantaged as Dennis would have been (taking over Dad's company). She would have had to fight to leverage those potential Hollywood contacts, pushed for auditions, trained hard to be recognised as an actress worth taking a chance on (unfortunately).
She was originally yes a 'buzzkill' but also the most level-headed and rational. She was a feminist. She cared about abortion and being pro-choice, enough to go to a rally! People are notoriously difficult to mobilise unless there is sufficient driving force or their moral compass is that strong that they feel compelled to show up and put their voice to a cause! That shows inner strength.
Then two decades of unfiltered misogyny rotted her brain and she became a pick-me. She likes 'being the only girl', she became desperate for validation, sleeping with arguably anyone who would look at her twice, drugging herself to get date-raped. Trying to actively compete with her brother in the pedophile olympics and shown not to have the same lack of morals as him!
Her anger is so vital to her character, I think. She has been beaten down and made so small and vicious and bitter. Her wasted youth and old age defining women's 'viability' and whether or not they are worthy of respect is a poignant look at how women are treated in society and how even if you align yourself perfectly to the axis of power, misogyny will always ensure you are crushed and sipped dry and discarded without a second thought.
I think in PTSDee, it all comes to head. That is the moment that she stopped being the gang's punching bag and realised she had autonomy - using it in absolutely horrific ways, but it's the recognition that she can be just as abusive and cruel and manipulative as the men, if not more so.
I think her anger comes from her mistreatment, but she'd rather die than turn it on the people who deserve it most: her friends, her father. They are her community. Every other woman hates her, rightfully so. She has no one outside of herself.
She makes me so sad. I love her so much.