The best way to spend as little money on criminals as possible is to have conditions that create as few as them as possible.
How to get there, that's a good question.
Personally, I think she should be locked up forever. She's clearly mentally ill. I'm inclined to punish those who aren't, yet committ horrific crimes, harder.
The difference, to me, is between fighting inner demons and losing, nor not knowing how to fight them, and just cold bloodedly deciding to murder another human in the worst way imaginatively, with little or no remorse.
YMMV. Just my opinion.