It wasn't because it was facing destruction, but because it was at the heart of it's power source.

At the precipice in Mt Doom, nobody could resist the temptations of the Ring. It's why Gollum was necessary.

A theme in the books is doing good only because it's the right thing to do, and another theme is evil often hurts itself trying to be selfish.

Frodo and Gandalf have that talk about Gollum, and Frodo says it's a pity Bilbo didn't slay him when he had a chance (in the books, it's before he leaves the Shire, in the movies, it's in Moria):

'But this is terrible!' cried Frodo. 'Far worse than the worst that I imagined from your hints and warnings. O Gandalf, best of friends, what am I to do? For now I am really afraid. What am I to do? What a pity that Bilbo did not stab that vile creature, when he had a chance!'*

'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 in 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 a chance of it.

That's my favorite quote from Gandalf.