Her boss told her that she had many more shirts to fold.
That sounds like a typical thing to do at a job. Do you think she should fold the shirts?
If it makes her unhappy, I do.  Personally, I want to ruin her life.
It sounds like she is fulfilling her basic duties at her job, which is expected of her.  It is wrong, though, for you to wish harm on someone for any reason.  If there is a problem between you, you should try to work it out rather than just taking pleasure in her misfortunes.
I can't stop thinking about it. It's like I'm obsessed with thinking about ways to make her miserable. I feel like I've become evil.
Being obsessed with making someone's life bad is not a healthy way to live. You need to find better ways to occupy your time and mind than focusing on making her life miserable. Find something that makes you happy and do that instead.