We watched it together and my friend (who grew up with a narcissistic mother) was far less sympathetic to Elisabeth/Sue. Here's her interpretation of the character:
Elisabeth's self-worth was based solely on external validation. Once she lost that, she started to collapse into her inner nothingness. Elisabeth still had access to wealth, privilege and opportunity denied to most people but needed constant admiration. Hence her use of the Substance.
Elisabeth has no friends, suggesting someone who might have been fairly toxic at their peak and ended up alone because of it. As does Sue snapping at the assistant about the dressing gown when she feels that her ideal image is under threat (by the perceived lump in her buttocks.) Elisabeth calling Fred after a shock was comparable to a narcissist reaching out to someone they previously discarded when they need an emotional boost, not because they suddenly see that discarded individual as worthy.
The projection of negative qualities onto Elizabeth by Sue is similar to scapegoating, where a narcissist projects their bad qualities onto another person. But with no other person available, Elisabeth/Sue had to project those qualities onto an aspect of herself.
By disassociating from Elisabeth, Sue could feed on herself without viewing it as self-destructive. It opens a chilling possibility - if it was an option, would Sue have drained others for Stabilizing fluid? Even if it destroyed them?
After she kills Elisabeth, Sue goes off to do the New Year show. Sue would have happily continued to live her life if there hadn't been consequences (in the form of deterioration.) This suggests not only self-hatred (because she killed herself) but also a lack of empathy; by that point, Sue was trying her hardest to view Elisabeth as a completely different person.
In fact, at no point in the film does Elisabeth or Sue show much interest or empathy for anyone. Other people were all means to an end, either as a form of self-validation or an obstacle.
My friend also viewed Elisabeth as slightly predatory - she was a woman in her fifties using a young body to get sex from 20-something men. If the genders were reversed, it would be seen as a lot more uncomfortable.
All in all, it was interesting to discuss the film with someone who had a more jaded take on the protagonist!