“Pity? It was pity that stayed his 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.”
“I am sorry,” said Frodo. “But I am frightened; and I do not feel any pity for Gollum.”
“You have not seen him,” Gandalf broke in.
“No, and I don’t want to,” said Frodo. “I can’t understand you. Do you mean to say that you, and the Elves, have let him live on after all those horrible deeds? Now at any rate he is as bad as an Orc, and just an enemy. He deserves death.”
“Deserves it! I daresay 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 judgement. For even the very wise cannot see all ends. I have not much hope that Gollum can be cured before he dies, but there is still a chance of it. And he is bound up with the fate of the Ring. My heart tells me that he has some part to play yet, for good or ill, before the end; and when that comes, the pity of Bilbo may rule the fate of many – yours not least.”
The Fellowship of the Ring (Boston: Houghton Mifflin, 2002) p. 58-59.