I am assuming the poison would affect the fort save for the death attack, if that save were failed. There are no rules on it.
The general guideline(not rule) is to apply offensive abilities in the order that is best for the attacker, IIRC.
I would say:
1. Death Attack, it happens during the actual strike.
2. Poison, it takes effect immediately after the strike.
Both poison and death attack occur as the result of a successful strike (the death attack is dependent on the attack dealing damage).
So I would do it like wraithstrike; the attacker gets to choose which applies first.
I thought defenses applied in the order best for the defender... (ex. Successful save then resistance). Truthfully I have a hard time believing the poison will have enough time to fully have an effect on you before you drop dead from being killed from a death attack that the poison is attached to but that could just be me...
Also I could be remembering incorrectly but isn't the first save for a poison to see if it even has a chance and saves after that are the ones for the effects of the poison?
The part about the defender is also true. As an example if the attack has a 2 options he can apply then in the best order. The defender then applies his defenses if any in the best order. <--That is now I remember it anyway.
As for poison you take the damage as soon as you fail the first save. You don't get a free round.
I agree that from a realism point of view that you would die before the poison would take effect.