The so called assassination principle I think isn't just a matter of black and white, right or wrong. It brings with it the matters of both moral principles, the ever present question of what would benefit society as a whole, and in the end when all is said and done were such actions justifiable, and to what end did they serve? Did they serve the purpose of protecting innocent people or did such actions only further the goals of what we sought to defeat and had declared wrongful and hurtful to our society?
Morality Fact: Most people can agree that it is morally wrong to murder someone, or take another life.
Problem with this Morality Fact: Today and throughout history, violent criminals, terrorists, tyrants, and warmongers have placed us in situations where taking another life was necessity. War however it was started, is unfortunate business and results in people killing people because each side believes in something the other does not, and each side believes that they are right. Today for the most part military operations are conducted primarily only in response to what pretty much anyone, anywhere would consider to be an act of terrorism or another country trying to impose itself on its neighbor(s).
My personal perspective: While I too believe, that under most circumstances it is wrong to take another life; there comes a time when taking another life isn't just about morality but about love, necessity, or the protection of your normally peaceful way of life. Granted this does leave a lot of room for extremism, but it is also said under the assumption that one is of sound moral character, and not a fanatic to some extremist cause.
So what about the assassination principle? To kill one and save a thousand... Again it isn't that simple. Even in the given example, knowing what Adolf Hitler does, who are we to kill a child? Now granted, here in the 21st century we know a lot about his life and his childhood, which may have influenced his actions as an adult, but does I guess the real question in mind here is do we have the right to judge a person based on such?
Cold hearted or not, the world is a better place without the Adolf Hitlers, the Bin Ladens, the Sadam Husseins, and if it means killing every terrorist, until the day they finally realize that society will not tolerate terrorism, then so be it. It is one thing to disagree with someone, it is another to believe in something another does not, but it is an entirely different thing to push past the boundaries and try to enforce your will on others. It's even worse, when you do it and claim it to be all in the cause of your religious beliefs. I may not be the most foremost religious person or expert on world religions, but I know of no credible, recognized religions that supports the use of terrorism (suicide bombings, car bombs, or just plain shooting people) in the name of said religion.
So do we kill that one to save a thousand? It all depends. In general theory perhaps not. But in cold hearted reality, perhaps so...if that's what it takes to ensure our continued way of life.