i think Duration = 0.00 will simply mean that the poison will forever remain on the unit unless removed via triggers or Dispel (just like any other buffing abilities). additionally, for Shadow Strike the Casting Time is dependent on Duration. if Duration is less than Casting Time, the Decaying Damage will not be applied, so i guess the idea that the "Initial Damage + Decaying Damage*Casting Time" reason why it became an instant kill is not true :O
maybe it has to do with the unit's other skills or triggers.