Is he a Muslim? Is that boy a Muslim? It would be foolish to let him live:
1) The boy will grow up and become just as ruthless a warrior and killer as his people.
2) Even if you save him, anger and hatred will grow in his heart for the humiliation you inflicted by saving him. Or do you think he can live knowing that as an adult, warriors will spit in his face and shout that he is a slave of the infidel who saved him?
3) The boy will die anyway. Without food, in the burned city, at six years old. From illness, cold, hunger, or the sword of another crusader.
4) It would be more merciful to send him to heaven; children's souls are pure, regardless of whether they are Christian or Muslim...
The second point – are you saying that the guy will live in the city of those he hates, and why would he kill his savior instead of these scumbags who are talking?
The third point – maybe he's not as stupid as you think, or the savior isn't so stupid as to abandon him to his fate after saving him!
The fourth point – and you, with a clear conscience, will continue to be an executioner – no, thank you!