You don't need to ID the asteroids at all.
And you're using a System compare to compare the asteroid damage value. The system doesn't know which asteroid you mean, so it's picking the first one, and watching that one. When that one goes kablooey, all the asteroids are destroyed because you don't have a sprite picked.
Use the asteroid sprite's own Compare Private Variable condition to check whether it's own damage is equal or greater to 100, and it works just fine.
You also need to "Remember picked objects" when you spawn your small asteroids with a function, so Construct knows which big asteroid to spawn from. Otherwise, when you kill asteroid 2, 3, or 4 then asteroid 1 will spawn the small asteroids because Construct can't remember which asteroid it was working with.
Also, you should check out the Custom collision mask for the astroids... with physics behavior you can draw a polygon around them to define a mask.