[Inziladun]

A very intriguing topic, Hookbill.

Personally, I would not consider Morgoth, Sauron, or Gollum as having "repented". I believe that only comes from a simultaneous recognition that one has done evil, with a sincere desire to pit evil away and do good for its own sake. That cannot be said for Morgoth and Sauron, whose seeming turnabout was the result of having superior force placed upon them by the Valar. Gollum too was affected mainly by fear, of Frodo, but more directly, the Ring that Frodo held.
It could be (and has been, exhaustively) argued whether Gollum's repentance would have held out if Sam had not awakened and accused him of "sneaking". My feeling is no; the Ring was stronger than any love for Frodo. That leads to the next question: was that Gollum's fault, or was he a victim? Both are true. The Ring's power was too great for him to overcome, but it need not have been so damaging for Gollum, had he not possessed an intrinsic propensity for mischief from the start.

Originally Posted by Hookbill the Goomba

Now, Boromir's fall is what gives me a strange thought. If Boromir gains redemption and can be classes as 'returning' to the 'good' side, it seemed to take his death to accomplish this. He had to go down fighting, and not easily, either. If there was redemption for him, it was not easy. Perhaps that is the point.

I would think Boromir's repentance genuine, however. As soon as he had chased off Frodo, he felt remorse, and decided to do what he could to redress his evil deed. That his death was necessary to repentance was apparently felt by Boromir himself, who told Aragorn

"I am sorry. I have paid."

Maybe Boromir was convinced at that moment that he had brought ruin to the world by frightening Frodo to a rash act that could have unknown consequences.

Much earlier, Isildur, upon being cornered by the orcs on the Gladden Fields, said to his son:

"Forgive me and my pride that has brought you to this doom."

UT Disaster of the Gladden Fields

Isildur "paid" with death also for his prideful error in taking the Ring.

To another example: would you consider Maglor's case as a "repentance"?
No, he and the sons of Fëanor were not "bad" in the manner of Morgoth or Sauron, but they did evil in obedience to a freely taken oath that they knew (or should have known) would lead to much chaos and bloodshed.

Yet, Maglor repented in his heart at least, and was willing to break the oath to avoid further slaughter, having sorrow for what had been done by himself and his brothers. It was apparently only the desire to go along with Maedhros, his last remaining brother, that caused him to fail

Notably, though Meadhros was moved to commit suicide, it was said that Maglor did not die, but wandered around aimlessly thereafter. Perhaps he saw that as penance he must perform?