I do not think that he was completly evil. Gollum usually was just tricked by the ring. And the reason he killed to get the ring was because of the overwhelming desire to posess it. Gollum did not hate everything. Remeber, there was a part of gollum's mind that was still gollum (or I should say Smeagol) and I think that gollum did want to change, even though it would take long. When he didn't have the ring, there were parts of him that showed that hadn't in ages. Also, even though he was quite bitter, when he didn't have the ring anymore, I think that a very (ok, not very but extremely) slow rehabilitation process began in him. You say that he hated the ring. Yes he did. He loved it and yet he hated it. This is the one thing that tore him apart. He loved it like a drug- he was addicted. But he loathed it. He had an instinctive hatred for the ring, and I think that's why he became kinder at times after he lost the ring. He realized how much he hated it and wanted to let it go but it was so hard. It was like a drug to him and very hard to quit, but even though it may not look like it, I believe that he was trying to quit; to let the ring go.
If you want to hear more about this topic, there is a thread about Gollum vs the other hobbits. Go here
web page