The One Ring and split personalities

[CaptainFaramir13]

It is true that Gollum's isolation could be a cause of the split personality. However, there are those instances where the Ring seems to cause a change in behavior in others. Perhaps these moments of the Ring acting on the darker side of the person is how it begins to create a new personality in them. If the Ring is creating a new personality I don’t think they would start referring to themselves in plural right away. But perhaps after years of changing personality between the normal-self and the Ring-influenced-self the Ring side starts to develop its own voice which is where the referring to oneself in plural thing begins.

[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.