The Keep is not stating "Kill the deserter outright with out obtaining the key" but only "Ostagar prisoner killed" and you can do that with any character depending on your dialogue choices. (You always get the key this way.)
You also can steel the key if your character is a Rogue, so all options the Keep outlines are indeed possible.
The Keep also outlines in only one outcomes that you got the key. So if you get the key in any of the other choices you made is not really of interest.
Ok, I can accept the part about killing the deserter then. But I've tried stealing the key as a rouge using the pickpocket skill butt it always says "Nothing to steal"
It has been a long long time since I played DAO as a Rouge.
Did the prisoner already admit to having the key when you tried to steal it?
I've tried both before and after he admitted to having the key.
