I don't think she's mentally unstable. I think she's cold, calculating and totally in control of her behaviour. She wants to make everyone else unstable, and it's working. The fact that it's working delights her, and that is why she is poison.
I think she's been through a tough time in her youth which has made her very protective of herself and of showing any weakness. So she retreats and sulks and isolates herself and refuses to engage in any conversation about her feelings because to do so would, in her mind, leave her vulnerable. When she's in a good mood she's vibrant and funny and I thought she was very sweet to nick during the letters from home task yesterday.
She's just taking the mick out of the others now for taking it all too seriously.
btw it wasn't the show which needed the warning about mental illness being talked about innappropriately. It's this forum which has been full of terms like nutjob psycho nutter and inferences about mental illness being linked to violence.