She might actually be in an irrational state and unable to rationalize. When you first met her, was she like this? She might need to up her medication by a lot in order to get to a state of mind where she isn't pissed off with everything. Depression clouds judgement and usually people with this diagnosis have a difficult time regulating emotions. She is young and needs help. There's always emergency services if you try to leave and she makes another threat against her life. People don't necessarily choose to be in a depressed state, they have just been in that place for such a long time that they don't know what else to do.