Scientific American has a great article about free will. Basically, most researchers generally lean towards a formulation that we do not have free will in the classic sense. Frequently, the argument that we "could have acted differently" is not true.
Unfortunately, these crude descriptions of complex freedom concepts have a strongly negative effect on people. Basically, they can't handle the truth. Nietzsche would be so happy.
Don't worry, everyone. Free will exists. The problem that we have is where the concept of "free will" exists. It exists above the underlying causes that result in "me." We can take this example to an atomic level and argue that everything that will happen is predetermined in the giant billiards game that is atomic interaction because causes and effects have predetermined courses of action. That still doesn't negate free will. Free will exists above atoms. It is a product of those atoms. On a human level, I do have free will to act however I choose to what I encounter. The fact that previous variables have an effect on that choice does not negate free will.
I am the culmination of every moment, both internal and external, in my life. Those moments must have an effect on future moments. I might choose to murder someone, say a black man, because I'm racist. That racism was caused by my family environment to a great degree, but just because it was the primary influence on my decision to hate black people and thus murder one does negate my choice at that time. Morals, much like free will, exists above the underlying variables that define a human. It was still morally wrong for me to kill the man. Having influences on my choice does not make the choice any less a choice.
Looking at humans as the culmination of everything that had come before also negates the Hitler experiment. If we define "Adolph Hitler," simply saying "the German fuhrer during World War II" is inadequate. We would have to define Hitler as "the man who existed who underwent every event as the entity labeled 'Adolph Hitler' by other people." That means that ANY change in his past destroys what we call Adolph Hitler and creates a new one. The new Hitler may be very similar to what we call Hitler, but it would be different. For example, our Hitler took a poop at 3:32pm, on October 22nd, 1933. If we go back in time and clog the toilet, thus preventing him from pooping at that time, we have destroyed Hitler.
Also, saying that killing Hitler would have stopped WWII is entirely wrong. Hitler got many of his ideas from the Thule Society, and Germany was filled with anger and poverty after WWI. In much the same way that a person is a conglomeration of variables, as is a group of people. Germany very likely would have gone to war, anyhow, just with a different leader. WWII would have been entirely different.
We are what we were. Choices are essentially comprised of things that have gone by, because what is a choice but the application of the past on an unknown future?
The words "free will" are nonsensical. What we want free will to mean is beyond the limits of language and understanding.
Choice and free will are necessarily above the underlying concepts that comprise a human. Interaction, past events, and neurophysiology are all parts of free will. We cannot apply it to its own underlying principles. For example, we describe things as "smooth" or "rough." We cannot use those words to describe the atoms that make up materials that we then describe as smooth or rough. Those words require atoms to exist.
Scientists say free will probably doesn't exist, but urge: "Don't stop believing!" (Scientific American)