Gandalf said that the Ring had nearly devoured him. I say nearly, because there was a little corner in his mind that was still his own, a corner not totally consumed by evil. Smeagol wasn't perfect before he got the ring, but the fact that even after 500 years of being consumed by the ring, he still retained a part of himself, speaks volumes. Bilbo was not corrupted by the ring, because of his innocence, the ring did not begin to really affect him until his Farewell birthday party. The ring stripped away Gollum's innocence, and corrupted him. There were times when he was his old self, not lovable, but pitiable. He knew what had happened to him, and he hated himself for it. Malte is right, the Ring created Gollum, and banished Smeagol into the back of his mind. there is not much written about Gollum's family, there is no mention of a father, but he did live with his grandmother. I view Deagol's murder as an irrational act, a spur of the moment thing, that defied common sense. With the Ring, he became a thief and a liar, he abused its powers, and eventually was kicked out by his grandmother for his misdeeds.
Nick