Jory didn't really die because of the joining as such, he died because he was a coward and Duncan was a no compromise sort of guy. Still not sure it was necessary to kill him.
I personally think that under those circumstances, it is easily justifiable (and necessary) to kill Jory. Throughout the mission in the wilds, he was constantly showing a weak will and would rather ignore his assignment and complain rather than get the job done. He got Duncan's attention by winning a grand melee in a tournament, but he was being let in on a lot of Warden secrets.
The fatality of the joining (which is a form of blood magic,) taking in the taint-making it so it's not an immunity so much as a delaying of the affects. He was also no willing to get over the whole "do what is necessary" part.
Daveth: The wardens do what they must right?
Jory: Including sacrificing us?
Daveth: I'd sacrifice a lot more if it meant ending the blight. You saw those darkspawn ser-knight. Would you die to protect your pretty wife from them?
Jory: I...?
Daveth: Maybe you'll die. Maybe we'll all die, but if no one stops the darkspawn, we'll die for sure.
Jory couldn't even answer if he would die to protect his wife. He saw the whole thing as, do a duty then go home and all is forgotten.