I understand Gollum’s attachment to the ring, but his lunacy seems to make him more deranged than deliberately evil. He certainly doesn’t have the same purpose for the ring that Sauron does.
With Gollum, think of an alcoholic's attachment to addiction. Sauron, on the other hand, works for Satan.
If you can, try to catch the replay of this program today at 1pm on EWTN. He explains many of the characters including Gollum and Sorum.