[Lotrelf]

Ring is an ultimately evil will and it tries to dominate every other will or power that comes in its contact. Bilbo also shows signs of split personality, when Gandalf asks him for the Ring; and those signs indicate him to be like Gollum. Frodo develops a spilt personality as well-- a hostile one somewhat like Gollum himself.
Ring works as a catalyst to your own evil/dark side amplifying it to be Evil. Bilbo, Frodo and Sam and Merry don't become Evil because of their inherent goodness. Hobbits' simplicity is another reason for this.