All your events are being checked all the time.
what you should do is something like this:
Give the enemy an instance variable Health
enemy : variable Health =< 0
On enemy destroyed
- system subtract 1 from varlifehearts
- LifeheartTiled set width : varlifehearts*24
Are all your enemies different sprites or are they the same enemy cloned?
For if they are cloned, I would recommend using copies rather than clones so they all respond to the same actions. (I'm assuming you don't have a paid version, for if so you could use families for that)