Star Trek: The Original Series, Season 1

Kirk:
[to the helpless Gorn] No, I won't kill you. Maybe you thought you were...protecting yourself...when you attacked the outpost. [louder, to the Metrons] No, I won't kill him! Do you hear? You'll have to get your entertainment someplace else! [helpless Gorn disappears, and a Metron appears] You're a Metron?

Metron:
Does my appearance surprise you, Captain?

Kirk:
You seem more like a boy.

Metron:
I am approximately 1,500 of your Earth years old. You surprise me, Captain.

Kirk:
How?

Metron:
By sparing your helpless enemy who surely would've destroyed you, you demonstrated the advanced trait of mercy, something we hardly expected. We feel that there may be hope for your kind. Therefore, you will not be destroyed. It would not be...civilized.

Kirk:
What happened to the Gorn?

Metron:
I sent him back to his ship. If you like, I shall destroy him for you.

Kirk:
[calmly] No. That won't be necessary. We can talk. Maybe...reach an agreement.

Metron:
Very good, Captain. There is hope for you. Perhaps, in several thousand years, your people and mine shall meet to reach an agreement. You're still half-savage, but there is hope. We will contact you when we are ready.