There's a lot to be said about Jenny in the L word, who is my favorite character and one of the best tv characters I've ever seen, but I'd summarize it like this :
By the end of the show, she reflects a lot of the worst things she has absorbed from those around her.
All the traits that make her horrible in the end are traits that can be found in each of the other main characters.
And she presents those traits without the charm that makes them palatable.
The fact that she meddles in everyone's lives, that she sabotages them, that she schemes, the fact that she behaves like a spoiled brat, the fact that she is hyper-controlling with her loved ones... That's how the L word group friends behave with one another.
The only person who doesn't see a reflection of herself that she doesn't want to see when she looks at Jenny is Shane, which explains why she's her only remaining friend.
She has become a worst-of version of Alice, Tina, Bette, Helena, and an (unflattering) embodiment of the group she hung out with for six years.
Jenny is the group's guilty conscience. Even her awful treatment of Max at the end of the series is the outward version of how the rest of the group treats him more discreetly - with annoyance and polite contempt.
So symbolically, it makes sense that she dies when the group frees itself from itself with the departure of Bette and and Tina, with the implied possibility that the group killed her directly.
It's the sacrifice of the scapegoat charged with the sins of the entire community.
I'd go so far as to say that those who don't like Jenny, don't like the main characters either, because she is them without gloss.
