When you are dealing with enemy health, you should always set up the death event so that it's like
When EnemyHealth LESS THAN OR EQUAL TO 0
Because the way things work, often you can "overkill" an enemy and nothing will happen to it at -1, -3, or -500 health.
Basically, a lesson you should learn is that when you code in C2 (or anything else) you have to think of ALL the possibilities in your game. In this case, you can't just think "What happens when enemy's health is 0? Easy, death." You have to think "Well what if he's shot a whole bunch of times really fast? What if his health is below 0?"