Typically things happen in the order from beginning to end. When this is not the case you will see things like "Before" show up and/or an Effect listed before the Hit.
If part of an effect is impossible, generally the effect is skipped.
Hungry for the Kill
Druid Attack 17
Encounter ✦ Beast Form, Healing, Implement, Primal
Standard Action ✦ Melee touch
Effect: Before the attack, you shift 2 squares.
Primal Predator: The number of squares you shift equals your Dexterity modifier.
Target: One creature
Attack: Wisdom vs. Reflex
Hit: 2d12 + Wisdom modifier damage. If either you or the target is bloodied or at 0 hit points or fewer, you can spend a healing surge, make a saving throw, or both.
Here the effect before is directly listed as before and above the hit box. And the check to see if you or the enemy is bloodied or dead is after the damage listing. The dead part would make very little sense if it didn't take place after the damage was registered.
In the case of your attack, the dead/unconscious enemy tries to make an attack, but can't because of the dead/unconscious state.