The BBC Magazine has an article on several "what if ..." scenarios on extremely difficult ethical questions.

This article was also discussed on Digg.

The examples in the article seem to too extreme, but there are real life examples that pose the same questions. For example, the policeman or soldier who has to kill people in their normal line of work. On a larger scale is how politicians and heads of state justify war on other countries to themselves and to the public. Remember when Tony Blair said that that war is for the sake of peace?

This is not the simple case of "lesser evil", but more complex situations that that.

Another somewhat related issue is moral relativity, which was first explained by Plato (quoting Socrates) in Republic, where giving back what is borrowed is a virtue, and a just person must do it. However, there are cases where giving back what is borrowed would be foolish, for example, the owner of a sword has lended it to you, but he has now went mad. In this case giving back the sword to him puts your life and that of others as well.

In this case the moral obligation of giving back what  is borrowed is no longer and absolute.