Logic and Devil
I'm writing this Oratory that disparages moral systems with absolutes, and I uncovered an interesting paradox. Consider the golden rule:
Do unto others as you would like done unto you.
Let's first assume that you are using the golden rule for your decision making. Here's a situation: You're a government official in Washington DC and it pretty much a certainty that terrorists will attack the city with an atomic bomb, likily severely injuring you. You have a terrorist in Guantanamo who knows information that will allow you to stop the attack. If you torture him, there is a significant probability that he will give up the information (let's say your torture methods have worked extremely well in the past).
Under the golden rule, there are two possibilities. The first is unrealistic: you like being tortured. We can discount this possibility because it's too absurd for a rational person (after seeing the analysis of the other option, you can work this one out for yourself).
The other, realistic, solution is that you are adverse to physical pain and torture. So you don't torture the terrorist and you are permanently maimed and mutated by the nuclear bomb. Knowing that this outcome would result when you made the decision, implies that you actually do enjoy being tortured (at least more than you like disobeying the golden rule). Already we have reached a contradiction, but it becomes even more ridiculous if we go one step further. Since we have established that you want to be tortured by not wanting to be tortured, it is your moral obligation under the golden rule to torture others. So, you torture the terrorist.
At the very least, what you want for yourself is at odds with your personal fate under the golden rule's prescribed action, leading to an indeterminant outcome. At the worst, you get the absolute worst-case scenario, where everyone suffers.

It's called guidelines, Adam. We pretty much abandoned absolute morality a long time ago, and went with relative morality.
Hence "the lesser evil". The phrase acknowledges that there is no good solution, just varying degrees of bad ones.