Since then, I've come up with another hypothesis. Whilst ILJ has enough mental issues to fill an entire chapter of the DSM, one of the most obvious issues appears to be Borderline Personality Disorder. BPD can turn its sufferers into truly awful people. Whilst I'm no psych, I've read enough to be aware that BPD is often brought on by childhood trauma and that it is more prevalent among females.
I'm not going to speculate as to what kind of trauma this may have been, and whether or not it was one trauma or multiple traumas.
I'm also not discounting my original hypothesis either. BPD sufferers can be volatile and manipulative, but I don't believe that BPD by itself would be enough to lead to alleged animal abuse, suicide baiting, prescription drug dealing, identity theft, incest fantasies, deplorable hygiene, attempted blackmail and/or egregious abuses of privacy viz. installation of covert security cameras (did I miss anything?).
One of the hallmarks of BPD is that they're desperate to have people love them and care about them, and are terrified of perceived abandonment. BPD manipulations are generally engineered towards that end - feigning a personality as they have an unstable sense of self and want to be loved. They'll often identify a "favourite person" who they place on a pedestal and become obsessed with, and at a perceived slight they'll switch to hating that person (sometimes with a back and forth between these two extremes).
There's some evidence of this with Bella's desperation for various women to love her.
However a lot of the shit she does isn't really the stuff you associate with BPD. Bella wants to feel superior to others, and needs attention and approval - that much is clear based on how she lies to her actual friends to paint herself as an elite hacker and a master manipulator. She acknowledged to Allen that she dislikes Louis and at this point was only pretending to like cats to continue her grooming of him, while planning to dose him with estrogen - and she's more than happy to throw people under the bus. She got off on manipulating Chris, she wanted to "steal" or "possess" Fiona, and she derived pleasure from exerting power over helpless animals (unlike a borderline, who typically would enjoy the unconditional affection of a pet and wouldn't hurt animals unless it was to e.g. get back at an ex - think boiling the bunny in
Fatal Attraction).
Borderlines also show remorse. They'll do terrible things to achieve their goals, but when they stop throwing their tantrum and cool off they'll generally feel terrible about it. They'll
probably play this up for emotional manipulation, but they will feel it. Bella showed desperation in trying to remain friends with someone after pissing them off, offering to buy them things - but not because she
wanted to be friends with them; it's clear from chat logs to others that she wanted to
maintain power over them, and was not talking about how upset she was that she ruined a friendship. When confronted, she downplays the severity of her actions or flat out denies them - despite being shown incontrovertible evidence that she did these things. Her tactic is to suggest that accusers are stupid or weird for believing these accusations, and that they've e.g. failed to understand the "intensely clever" manipulation tactics she was employing against Chris.
All of this makes me think she isn't borderline or histrionic; it's how a narcissist acts. Her chosen way of viewing herself as superior is to see herself as a fiercly intelligent black hat 4chan oldfag - she can hack anything she wants, she can control and extort a famous figure from internet history, she can target and humiliate transgender people, she can abuse animals with impunity, she can seduce heterosexual women, she can convince an entire forum to pursue a different target. However she doesn't seem as fragile and sensitive to criticism as a narcissist, and she's also very impulsive, aggressive and self destructive (lacking awareness of the consequences of her actions, and repeating the same destructive patterns of behaviour - in this case, socking over and over and continuing to use Discord despite it continually getting leaked). That would point more towards ASPD, or ASPD comorbid with NPD, although you'd expect to see some evidence of a conduct disorder in childhood/adolescence.