[The Saucepan Man]

Gollum is a pitiable character. Not truly evil, but corrupted by the Ring. He is totally driven by his desire for it. It is this that makes him act in the way he does. He wants to kill Bilbo for stealing it from him. And, when he is with Frodo and Sam, his purpose is to keep the Ring from being destroyed and, when the chance comes, get it back. At first, with Frodo and Sam, he settles for being close to it and, for a period, is moved by the kindness that Frodo shows him.

Ultimately, however, his desire is the Ring and this desire was always going to prevail in the end. I suspect that, throughout his journey with Frodo and Sam, he was merely biding his time.

I have always thought that Gollum first hatched his plan to lead them into Shelob's lair when they were before the Black Gate (or possibly before), rather than when he feels betrayed by Frodo at the Forbidden Pool. After all, he suggests that they go through the passage at Cirith Ungol knowing that it is Shelob's lair. He might kid himself that he is Frodo's friend, but deep down he wants that Ring.

So, pitiable, yes. But he wouldn't be my first choice of travelling companion if I was the Ringbearer.