So, if I knew absolutely that it would doom the continent etc., I would have to kill him. That said, I really can't know this. If it were the only instance, perhaps, but there are plenty of others. Still it makes for good conversation.
The point wasn't whether or not this would actually happen, so much as whether or not you're willing to acknowledge that the very basic idea of killing to save others lives is justifiable. If you don't, then of course the Wardens are not justified in a lot of what they do. If you do, then it gets to be murky. A lot of the Wardens actions, while repulsive, probably do save lives. And if Jory were to tell everyone what the Joining was and why Daveth was dead, that would not be good for morale in a battle that was already looking bad. And it's hard to keep him quiet without killing him. He'll probably tell everyone he meets, and you can't very well keep him away from literally everyone who is not a Warden.
I suppose that even if you do acknowledge the basic idea, though, it is all a matter of where you draw the line. The little shepherd boy, for example, is a little bit unfortunate even if it gets to be as unambiguous as I made it, and complicating factors do just that.
My suspicion is that the secret is already out. The Ash Warriors appeared to know it as did the dwarves, and the effects are well discussed in the game.
I suspect that the very highest ranking members of the Templars know it as well. They would probably be interested in knowing why the Wardens can do the things they can unless they already knew and had a reason not to make an issue of it.
As for the dwarves, they wouldn't care much. They respect the Wardens that (arguably) use Blood Magic, and don't care for the Chantry that forbids it. Anora knows, but assuming that her own University is built on the Orlesian model it would weaken the Chantry's grasp on knowledge, so that's weak evidence that she doesn't care much for their restrictions. Though to build on Anora knowing: while Maric was apparently known to have been deliberately kept in the dark about a lot of things at first, and Loghain certainly was, I remember reading a fan theory that the usual arrangement with heads of state is to let them know the important Grey Warden secrets, partially because they need to know not to interfere and to prevent others from doing so. (Which, again, some people totally would. Leaving aside how serious the Wardens would lose if it were known to the uneducated and to fanatics like Rylock, they would lose something.)