Well, I don't think there's much debate that Rose and Lissa would both have died if they'd run into a Strigoi. Rose was out of shape and didn't have a stake nor any idea how to use one. Dimitri was completely right when he pointed out that if she couldn't handle her classmates fighting her, she would die against a Strigoi.

But I do think that Lissa's mental instability required some drastic action. Lissa had been going into a downward spiral since the car accident, and Rose knew that. Between spirit, her new popularity with the backstabbing 'in' crowd, and becoming an orphan and only child at the age of fifteen, Lissa was going to crash soon.

Overall, it was a sucky situation. And I think Rose did the best she could with it. Besides, with Mrs. Karp's lingering compulsion effects, she didn't have /that/ much of a choice.