Makes sense i guess? Kill the girl and let a child of the king and that woman become heir instead.
btw, it would be moronic to hand him over, all are going waddadada about how it could be seen as an attempt from this country trough that behaviour, but all are just ignoring that it could be an attempt of the visitors country to give them a war reason, so they take the boy home, hang him and then just say that he confessed to be hired by the country to kill the princess.
It is ALWAYS a incredibly dumb move to give a prisoner in a incident that involves also your own country to give to another one...
The making him a servant is sure debatable, at least i wouldnt tell them.