It is so because at the moment you have a global variable acting as the "health counter".
Once this health counter is null (0) you destroy all the instances "nemicosinistra" (not a peculiar one).
To solve this, your health counter should be an instance variable of "nemicosinistra".
When you test if the instance variable "health" of the object type "nemicosinistra" is equal to 0, then C2 will pick the instances which "health" variable is equal to 0.
For this you can check back the beginner's tutorial
which clearly demonstrate this principle in page 6