Flaws in Lord of the Rings... Yes in the book.

A few thoughts on your post.
I am not sure how you could see the One Ring or Sauron as anything but evil.
The Ruling ring was forged in secret to control all the other rings. It was filled with Sauron's wrath, malice and will to dominate. The power of the ring and Sauron for that matter are considered evil not because of the power they have but what the power is explicitly used for-Domination, destruction and cruelty.
I don't think that Boromir was necessarilly evil. I think he was more "flawed" He wanted to destroy the evil that was menacing his homeland but in his willingness to stoop to whatever level was necessary to do that; he was blinded to the consequences of the ring's use and to the true evil nature it had.
With regards to Gollum I think he was somewhat evil to begin with and by the time of the events of the Lord of the Rings he is more "insane" than anything else.
Finally, in regards to your last statement about people in Middle Earth not being willing to see Power as something that can be used for good, I am not sure that is expressed in those terms. If it is, I would say it is a healthy scepticism for anyone or any group that can weild enough power to change the course of events across nations and worlds. One always has to be suspect of the motivations of said groups or people and what their power to change things means to each of us as individuals pursuing our own goals and growth