I have read the book. The themes that I think may be evident in this passage are probably ones of divine providence, chance, fate, something like that. The goblins wanted to kill Gollum, but Bilbo spared his life. They did not like this decision obviously, but it turned out that it was the right decision for biblo to make. If he had not spared him, they would all be killed and middle-earth destroyed. Gollum was almost killed several times, but he was always spared by some unseen thing. A god, fate, chance, who knows? I'm not really sure if this totally answers your questions, but it's something. You'll at least get half a mark for trying, right? lol
