Re: Stupid but?
Prison seldom works as a deterrent. Extra long sentences never do. If you're willing to give up two years of your life living in a box and have every aspect of your life controlled by prison guards, then you're already not thinking rationally about cost-benefit. So why do you think twenty years would make someone suddenly rational.
It won't, but she'll be at least ten years older (time off for good behavior), probably not very cute after eating prison food for ten years, and a felon, which will make her so unemployable as to make her a lot less hot, even to a depressed person with low self-esteem. All of which may blunt her ability to make someone else's life miserable again.