There, Robert, you have kill'd that fly — ,
And should you thousand ages try
The life you've taken to supply,
You could not do it.
You surely must have been devoid
Of thought and sense, to have destroy'd
A thing which no way you annoy'd —
Lamb is angrily ranting towards Robert because he has killed an innocent fly. When he states, “The life you’ve taken to supply, you could not do it,” he means he has taken that poor fly’s life and will never get it back no matter how hard he tries. 
 

Thoughtless Cruelty