I think Gollums motives were simple;
1.Kill Samwise-
In Shelobs lair he attacks Sam and not Frodo, maybe this was because he said he wouldn't harm Frodo but, please correct me if I'm wrong here, but I thought Tolkien described Gollums attack on Sam as "spite", doesn't that mean he intentionally did it?
2.Eat fish-
I think Gollum said once that his main dream was eating lots of fish or something, so when he got the ring don't you think he might go somewhere else to have some fish?
