Sorry to come into this a bit late but..
Not important, it is merely example evidence. Whether or not it is supernatural has no impact on its power.
(this is in refrence to an assassin's Death Attack ability being stopped by Death Ward)
Incorrect.
"The subject is immune to all
death spells and magical death effects. The spell does not protect against other sorts of attacks, such as hit point loss, poison, petrification, or other effects even if they might be lethal."
As an assassin's Death Attack is neither a spell, nor magical in any way (hence the Ex tag), Death Ward would not stop it. Much in the same way Death Ward could not stop age from killing you, a nasty fall down the stairs from killing you, or a coup de gras from killing you.
Additionally, looking over all the various spells that do have the [Death] descriptor, they all seem to operate on directly destroying life force. Instead of, say.. scaring you really bad (Phant. Killer), burning you to a cinder (var. fire spells), evaporating all the fluids in your body (Horrid Wilting), transmuting your body to stone (Flesh to Stone), or discorperating your physical body (Disintegrate). You cannot Raise somone who has been killed by a Death effect because their soul (life force) has been destroyed. Similar to why you cannot raise someone who has been turned into undead: when you die, your soul goes free to the afterlife ... when you're brought back as undead your soul is forced to reinhabit the body and bound as it's animating force again. Bound being the operating term.. destroying the body, destroys the soul.
So, examples.. cause examples are fun.
Illusionist John: *casts Phantasmal Killer at Farmer Jenkins*
~Phantasmal Killer manifests in Farmer Jenkins mind as his worst fears: his now passed-on mother~
Phantasmal Mother: "Boo."
Farmer Jenkins: "Oh no! My mother!"
~Farmer Jenkins' heart stops from the sheer terror of the apparition~
Farmer Jenkins: "Ack, my heart!"
~Farmer Jenkins dies~
In the above example, Farmer Jenkins is killed by fear. His life-force remains intact, and he could be raised from the dead if his soul wished to come back, and the appropriate spell were cast.
Example 2.
Nerull Nun Nancy: "Die for the glory of the Rotting Lord!" *casts Slay Living on Farmer Jenkins*
~black, unholy power courses from Nerull Nun Nancy's palm, and invades Farmer Jenkins' body~
Farmer Jenkins: "Oh no! An evil priestess!"
~Farmer Jenkins' soul is consumed by the dark power Nerull Nun Nancy commands~
Farmer Jenkins: "Ack, my soul!"
~Farmer Jenkins dies~
Now in this example, Farmer Jenkins is killed by having his soul / life-force destroyed by the Slay Living spell. Said life-force is no longer present
anywhere. It's gone. Period. As such, Farmer Jenkins cannot be raised back to life without the application of a Resurrection, True Resurrection, Wish, Miracle, or at the GMs discretion, a Reincarnation spell.