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 —
You'll one day rue it.
Twas but a fly perhaps you'll say,
That's born in April, dies in May;
That does but just learn to display
Robert must have been mentally imbalanced for destroying the fly because the fly did nothing to annoy him. He also says that one day he will regret it. Is killing a fly such a big, regretful mistake as Charles Lamb portray’s it to be? 
 

Thoughtless Cruelty