Frodo: What a pity Bilbo didn't kill him when he had the chance.
Gandalf: Pity? It was pity that stayed Bilbo's hand. Pity and Mercy: not to strike without need. And he has been well rewarded, Frodo. Be sure that he took so little hurt from the evil, and escaped in the end, because he began his ownership of the Ring so. With Pity.
Frodo: He deserves death...
Gandalf: Deserves it! I dare say he does. Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death and judgment. For even the very wise cannot see all ends. I have not much hope that Gollum can be cured, but there is a chance of it.
From J.R.R. Tolkien, in Lord of the Rings