One of the many themes we spot on LOTR is "despair".
Basically, "despair" means hopelessness, loss of hope. People who are desperate believe that there's is no reason to live for, that they've lost everything in this world and so they hate their lives. They maybe even hate themselves.
On LOTR despair is clearly reflected on two characters: King Theoden of Rohan and Denethor, the Steward of Gondor. They are both old-aged rulers and they both lost their first-born sons, therefore they believe that their line has ended and there will be nobody to carry on their legacy. Also, they both have other people to take their place (Theoden has a nephew and Denethor a second son) but they don't value them. And, in addition to that, there's a great war they must face, against a strong enemy.
Those men are overrun by despair, both of them. But there is a difference, you know: Denethor is driven mad by despair and commits suicide (sets himself on fire), while Theoden leads his army one last time against a superior force and manages to save the great stronghold and buy enough time on the battlefield until help arrives. Eventually he is killed by a stronger foe, but not before he has performed his bravest and greatest act.
Despair can lead someone to either commit suicide, or perform the greatest and highest deed of his life. One final act of bravery, valor and selflessness. One final atonement...