So you are just anti-military and are preaching passivity and surrender.
To review: you asked, "Is it wrong to slit the throat of a 16 year old boy who is napping, if you are on your way to kill a terrorist?"
On the face of it, you are asking whether it is wrong to intentionally kill a person who is, atr the time, helpless, and who is not aggressing against you. (What you're on your way to do, is irrelevant; the boy's age is irrelevant; and in your description there's no reason to think he is an aggressor.)
It's a peculiar question.
Slitting the throat of a sleeping adolescent is not an act of war at all, let alone an act of "just" war. It's merely murder.
Even if the sleeping person were a IED-planter, the fact that he asleep would presumably mean you have a way of stopping him without killing him. You have a moral obligation to stop him. Tie him up, surely. Throw him in a locked room, surely. Break his arm, possibly. You do not have a moral right to slit his throat.