Vader was never known for mercy.

In his very first act as a Sith Lord, he murdered hundreds of Jedi Knights, younglings, and force adepts in the temple on Coruscant.

Since that moment, even when the objects of his wrath were innocent, Vader showed indifference to whether they lived or died.

So if he had been so heartless in his murder of younglings, why did he let this particular teenage girl live… even after she had tried to assassinate him?

To know the truth, we have to start at the beginning…