Sally has been abandoned and alone all her life, and Bob was the only one who made her feel loved and special.
Bob is a Manipulative Bastard who passes himself off as a nice guy, and maybe even honestly believes he is one. Over the course of his relationship with Sally, he's gotten her to center her entire life around him, which has in turn caused her to become a Clingy Jealous Girl. Then Bob—possibly oblivious to the fact that he ought to have expected this outcome—dumps her for Alice because Sally's become a Clingy Jealous Girl.
Sally is a psychopath and a narcissist. As result, she needs to feel loved to maintain her high self-esteem. Bob is the only person who loves her as much as he did and because of this, she is willing to go to insane lengths to keep him all to herself.
Sally is normally nonviolent and sane, except around Bob, who seems to be a walking trigger for her.
Sally is normally sane, but acts insane, as she wishes to be hated by Bob. One day, Bob says he doesn't hate her, at which point Sally does anything to get Bob to hate her.
Sally, a butcher's apprentice, is rejected by Bob but takes it rather well. But whenever Bob sees her after rejecting her, it's always with a bloody axe and a grin from the meats she's cutting up - and he panics for the imagery in his own mind.
Zig Zagged: Sally's views on what Bob can or can't do seem to depend greatly on how she's feeling at the moment; she may declare that she wants her beloved to be happy in one episode, only to crack with the pressure the next episode and almost kill Alice before realizing what a terrible thing she was about to do to Bob's happiness; in the next episode, she witnesses a fight between Bob and Alice and decides that he'd be better off without her...
Though jealous, Sally does not consider genuinely insane solutions to getting between Alice and Bob.
Discussed: "Have you ever noticed the way Sally's eye starts twitching when Bob talks to Alice? It's really creepy."
Conversed: "Oh great, this show has a love triangle. These things almost always end with the odd one out either gracefully giving up or boiling a rabbit. And just giving up doesn't exactly make for drama, so we might as well say goodbye to Flopsy right now."
Sally's instability is due to Bob keeping a Betty and Veronica situation going on for too long without considering either girl's feelings beyond "Awesome, two chicks dig me!".
The only thing that renders Sally insane is her fear of abandonment, and even otherwise, Danny insists she should be treated with compassion. He comforts her, and she succeeds in her attempts to overcome her abandonnement issues, avoiding to have been slowly driven completely mad. She drops all violent behaviour, realizes that Alice is the best for Bob, still wants her beloved to be happy, and retreats. Even Danny and her might get their happy ending... together.
Bob dumps Sally after he finds out about what she did to Alice.
And when Bob finally gets around to picking Alice, Sally's unrequited feelings cause her to Murder the Hypotenuse.
Except Danny is now jealous and can't bear that she has become geniuely Just Friends with Bob. She can't bear that he did the same with Alice. One of them ends up cooling down. The other creates unsettling fantasies to entertain oneself. What ifa minor accident...
Played For Laughs: Bob has no clue that Sally loves him; she makes several cartoonish attempts to break them apart, only to push the pair closer together.
Played For Drama:
Only the audience is aware of Sally's instability, and watch it develop and worsen as Bob and Alice grow closer, unaware that every sweet moment they share drives Sally closer to the edge.
Alice is Sally's best friend. Sally feels ashamed and guilty for wanting to harm her best friend and her love interest, but with each nice thing Bob does for Alice she finds it increasingly difficult to keep such thoughts away.