The notion that “if I would be in his place, I wouldn’t do that” – is completely wrong. If you would be someone else, you would do completely what he did.
Nevertheless, believing in “free will” is substantial for our civilization, society and so on. We as humans cannot exist in any societies without assumptions that free will exist. Saying that, we are at the same time distributing it uneven among different people to exert the impression of responsability. Some people are more responsible that the other.
Moreover, the idea of treshold come to mind when we are thinking about responsibilities and punishment. You can be judge when you turn 18 years. You cannot take responsibility if psychiatrist conclude you are crazy (mentally ill). There is this distribution of “normal” (healthy) and mentally impaired people, one that are responsible and one that aren’t responsible for their actions, respectively. But the treshold in not fixed, and it is changing constantly. The true reason that it is not fixed is come from bas assumptions of free will, namely, the notion that such a thing exist.