She didn't need to do it.
Yet she grabbed the cup, put it inside a small wooden box, and hid it along with other stuff in a bag.
She didn't have to do it because it was not only wrong but also unnecessary. The cup had no value, it gave her no benefits. And after she tried to escape with it, she was quickly found, arrested, and put in jail for the rest of her life.
On the third day in jail, a man said to her "I am here because I protected my family. I did everything right to keep them safe, and I'm paying the price. But you gained nothing."
"So?" she answered.
"So why did you do it then".
She looked at the sky, it was cloudy. It looked like it was about to start raining.
"Because I could".