That's interesting, but I think it's bugged. Death ward does normally block negative energy, at least in all of the content I've been playing. Have you had the same experience outside the TOR, such as the Eveningstar quests?
This is my experience as well.
The enemies in Tor have Negative Energy protection, but not Deathward. At one point they might have had Deathward as well (if memory serves, but that might have been the old Epic Ward as well, so can't say for certain), but that is no longer the case anymore. You can cast Enervation on the Giants in Tor, and they will have a Deathward symbol trigger over them (The protection from undue harm effect) and block the Negative Energy damage over time, but not the Negative Levels.
If you cast Rend the Soul or Necrotic Blast (without any Epic Destiny bonus damage such as from the Mantle from Draconic Incarnation), you will see that they take zero damage from it. However, the damage from Epic Destiny Mantle, such as that from the Draconic Incarnation Mantle (including Shadow Dragon for Negative Damage) ignores such protections on the Giants in Tor and will deal damage as normal (even though it is Negative Damage in the case of the Shadow Dragon).
Otherwise, Deathward spell will prevent the Negative Damage and any instakill effect. Negative Damage or otherwise.
J1NG