I find the existence of free will implausible. My reasons are as follows:
Mind-body dualism, that is, the belief that mind is something separate from the body, seems implausible in the light of this. The man had a brain injury, and his entire personality changed.
Given this, what mechanism could one expect to find in nature that would explain free will? I suppose at this point people like to invoke quantum mechanics, but I don't understand quantum mechanics and I don't know if the people doing that do, either.
Free-will is every bit as unfalsifiable as a belief in God. Nobody can prove free will doesn't exist through empirical means that can be replicated, so it should go out the window, if God goes out the window because of such a standard.
Moreover, personal experience and analysis, while not perhaps verifiable, suggests that people always make their choices based on past inputs, whether they are aware of it or not. There is no randomness to it. There is a certain level of predictability, although I should qualify this by saying that much of the time, when we interact with each other, we are interacting on the basis of incomplete information. Because the information is incomplete, the choices appear more nebulous and somewhat "random."