I once trapped a fly in a cup for 14 hours
Why would you do that to a fly? It doesn't seem fair.
Fairness is something that's not in nature, it is a human invention.
There's a well-understood rule that mostly everyone follows where you don't hurt anything that can't hurt you. How would you feel if someone much larger than you captured and starved you?
I bet that fly is dead by now.
The fly probably is dead, and most people wouldn't think that is a good thing. It's considered pretty lame to go around killing things that can't defend themselves.