I never checked if DamageEngine's AfterDamageEvent (with fatal damage) will fire before or after the unit actually is dead. I would imagine the death event will fire afterward, but I am not positive.
If DamageEngine were used for this, the trigger would look like this:
Events
DamageEvent becomes Equal to 1.00
Conditions
Owner of DamageEventTarget equal to Neutral Hostile
Life of DamageEventTarget - DamageEventAmount Less than 0.41
Actions
Unit - change ownership of DamageEventTarget to Player 12 (Brown)