I don't think Gollum was really evil, like Sauron, for example. And yet it seems to me that he is the only one of Ringbearers who got the Ring by murder (Sauron made it, Isildur cut his finger, Bilbo found it, Frodo inherited it, Sam took it from Frodo and only Gollum killed his friend to take the Ring). Is he not evil after that? But I think his evil is somewhat minor compared to Sauron and Saruman and other bad guys whom we know from Silmarillion.
I think Gollum just forgot how it was to be good. He has never seen much mercy, has he? I think lots of love (and fish) would have been the best cure for him, but everybody in the book was either unwilling or too busy to give him love and sympathy
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