Well, you CAN break the nose in such a way that fragments could potentially get into the brain. You have to first do a side strike (as in a backfist to the bridge of the nose) - -that breaks the bone, at which point a palmheel would send it up and backwards. Problem is I have never seen evidence that this would KILL someone- - at best it might remove their sense of smell or some higher reasoning functions. You'll be sending these bone fragments into the base of the front and bottom of the brain. The olfactory tract and olfactory bulb are there so it's possible you could sever the olfactory tract and remove their ability to smell things. I suppose you could also hit the cortex at which point they might have some psychological programs- - -but the stuff that regulates life itself - -heartbeat, breathing, etc. . you'd have to drive those nose fragments way back into the rear of the brain - -to do that you'd have to get a sledge hammer and bash his face in. . .
I think this myth arose from the idea that if you stab something into the brain it WILL cause death. That idea is wrong, as a British railroad worker was good enough to prove when he somehow managed to drive a railroad spike through his head. He survived, but the spike went through the areas that control personality, and from what I've read it turned him into a royal jerk.