Is it not obvious? You clearly know who he is. Or was, if he's dead. You could return it to him, or his grave. The woman I am is not fit to hold that dagger or meet the man that boy has become. It is better if you do it.

[Or maybe she's afraid that boy became just as twisted as she. Did he die in the Kingdom at a young age? Did one of her soldiers slaughter him as a nameless enemy at one point? Or is he alive, and well, and possibly resents the woman in red for throwing Fodlan into war.]