One of the darkest chapters in Elven history was the Kinslaying, when one faction of Elves unjustly warred upon another to capture their ships, and leave the Undying Lands and the divine powers against whom they had rebelled. For this, they were exiled, and most never returned to paradise. In Arda, the universe in which Middle Earth lies, one of the most grievous sins is the killing of one's own kind. Those who attack unjustly are the most guilty. Those who must kill in self defense, however, suffer as well.
Professor Tolkien experienced the horrors of war before he "sub-created" this little history. He understood that even just wars were ugly, and like the Church he held so dear, he professed that the best course of action is for killing of one's brethren to be avoided whenever possible. Hence Frodo's insistence that Hobbit blood not be shed during the Scouring of the Shire (a chapter which, unfortunately, Mr. Jackson left out of his film entirely). We should feel how tragic it is when one human being takes the life of his brother, even if it was to justifiably defend his own. In an ideal world, such things would not even be necessary.
As do the peoples of Middle Earth, we live in a fallen world where a few men become too attached to power, and so greedy that they suck up as much of it as they can, and abuse it in the most heinous ways. Such tyrants terrify us with the sheer magnitude of their crimes, and justifiably we are angry that they are able to perpetrate them. And sometimes we are forced to take their lives ourselves, or the lives of the men in their armies, because we must protect the world from their dark will.
But we should not allow our anger to let us feel good when we must remove them from the world ourselves. Such tasks should disgust us, and we should take them on only when we have absolutely no other choice. When we can learn to take the life of another human being gleefully, uncontainable despot and incurable criminal though he may be, we should fear for ourselves. It is a short step from this, to learning to take the lives of men less criminal than he out of anger and revenge, and eventually, when our hearts have become hardened enough, even ending the lives of the innocent for our own convenience. Our just God also offers us mercy, and we are called to make both Justice and mercy a reality in our world whenever it is humanly possible.
Justice, revenge, distasteful necessity, and selfishness must not be confused with one another.