[Hookbill the Goomba]

Pride and despair! As Denethor said. These seem the great downfalls of people in Middle Earth. Overcoming them appears nigh on impossible for a lot of the examples we've shown here.

Originally Posted by Boromir88

"My pride has fallen," and it sounds like Isildur intended to give up the Ring. Whether he would have done so had he ever gone to Imladris, well, won't be known. However, he was urgently in need of Elrond, and his pride had now fallen. Sort of puts an interesting spin on the whole "ambush" scenario...a sentient Ring knowing going to the "Keepers of the Three" would not be an ideal situation? So, let me draw evil in to kill this guy and get lost in a large river for a while?

This certainly seems plausible. It would tie in quite well with Aragorn's own reluctance to attempt to take the Ring, if he'd known that Isildur had fallen to pride and he did not wish to come to the same fate.

Originally Posted by Inziladun

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?

The sons of Fëanor again seem to display this trope of pride, and a little despair.

Originally Posted by The Silmarillion

"If none can release us then indeed the Everlasting Darkness shall be our lot, whether we keep our oath or break it; but less evil shall we do in the breaking."
Maglor speaking of the Oath of Fëanor

But the oath seems to have sealed them in. They must shatter their own pride in order to break the oath, and so achieve less evil. Maedhros falls to despair, much like Denethor, and burns in the mountain. Maglor settles for, as you say, Inzil, wandering aimlessly. In both cases, if this is redemption, then it is hard bought.

Originally Posted by Rumil

I do have an example, but not exactly one of the greatest or most puissant characters of Middle Earth!!

Lobelia Sackville-Baggins

Now as we all know she starts off mean and spiteful and money-grabbing, determined to wrest Bag-End from the Bagginses and add to her silver spoon collection. After Sharkey's ruffians took over the Shire she 'went for' one of them with her umbrella on being insulted and off to the lockholes they dragged her.

On her release she was cheered by the crowd, became popular, but was crushed by the loss of her son, she returned Bag End to Frodo and retired to Harbottle. Dying the next year, she left all her money to help hobbits made homeless by the troubles.

I'd say that was a pretty good turn-around

Interesting point! Perhaps she is not a villain, but she is an antagonist. She, too, seems to have some degree of pride. There is this sense of wanting to be recognised in the community, also a desire to have what she believes is hers, rather like Gollum's birthday present, or Isildur's 'payment' for the loss of his father and brother. She is 'redeemed' again through hardship; she sees her lands ruined, she is thrown in prison, and eventually, the crowd cheers her. She finally has the social recognition, but perhaps not the kind she had looked for. However, she seems humbled by it, and retires from the grudge match with the Bagginses.

Originally Posted by Inziladun

Might we also consider Thorin Oakenshield?

Again, not really evil; more, like Boromir or Gollum, a character defect that can lead to evil.

In Thorin's case, his avarice for wealth was not merely personal, but a hallmark of his race.
Still, upon his deathbed (again!) he found it in his heart to say to Bilbo that such pursuits as money and jewels should be renounced in favor of more simple, hobbit-like pleasures.

I think this is probably the closest we'll get! Like all the others he is brought to 'evil' by his pride, his lust for what he sees as rightfully his. But as you say, he too buys his redemption with death. Not only his own death, but of many others, including some of his company.