[Lord of Angmar]

I think we overestimate Sam's influence on Gollum/Smeagol

I agree. I think, as Imladris said, that there are several different influences on Gollum that can be deduced as causes of his return to his old, violent and lustful ways. I do not believe, as was suggested in the movie, that Gollum every wholly repented of his craving for the ring or his rotten ways, but rather that it was hidden. It was only inevitable, I believe, that as they drew nearer to the stronghold of Mordor and the fastness of Barad-dur that his old ways would come forth more, being influenced by his mistrust of Frodo after the incident at the Forbidden Pool, by the will of the Dark Lord, the growing power of the Ring, and, in part, by the taunting of Sam.