..so somebody who decides to plough into a pedestrian and kill them while driving,
presumably has a reason why they do that. They could have decided to put on the brakes.
The only reason that is acceptable, in law, is that the person is insane.
Any other reason, while possibly true, has no bearing on the fact that they decided
to kill somebody .. and that is what I understand to be exercising our free-will.
Maybe the person "lost his/her mind" for a split-second .. makes no difference.
Blaming the devil or evil thoughts is not acceptable, even if true. We are expected to control ourselves..
..or forfeit the right to drive etc.
Even if we have no true free will, it doesn't mean that we shouldn't be held responsible for certain actions, because in many cases we can suppress our emotions that could lead to such actions. In the case a person is physically damaged, like an insane person which feels no real remorse for their actions, we do not hold them accountable in the same way as a person who is considered sane, which is why they usually go to certain facilities able to deal with them.
I would argue that a person deciding to plough into pedestrians on purpose is not sane if there is no motivation for doing it. A good example I think is terrorists where we have seen many examples of them doing such things or deciding to blow up people, I wouldn't put them in the same category as a physical damage person, but rather people who are highly motivated/convinced by a certain idea.
But even the terrorist you can probably track back to how they ended up doing what they did. In many cases, I don't think these people were born with the purpose of ultimately ending up as terrorists, but things in their lives simply turned out in such a way that they got convinced that doing it was the right thing to do. For instance, there seems to be a tendency that most terrorists are related to some specific understanding of Islam, whereas we don't really see a lot of terrorism going on in Christianity or Judaism or people motivated by atheism. Don't misunderstand me as saying that Islam = terrorism, because obviously, it is far from all that do it. But I would make the argument that a potential terrorist that was brought up in another culture or religious view, probably wouldn't have ended up as a terrorist, due to the mere fact that the experiences and environment would have played out very differently.
And you are correct there is a degree of responsibility involved as well, but humans have a lot of emotions and likes and dislikes, which we have absolutely no control over. For instance, you didn't choose to be friends with person A, because you decided using your free will that the person was nice, but for some reason, you and him/her simply got along and then ended up as friends. And I think that holds true with a lot of the things we end up doing, using the example of driving from the last post, maybe you want to drive to your friend's birthday because you emotionally don't want to disappoint them, even if you don't really feel like going, you will make a for and against whether disappointing your friend is more beneficial for you than simply going there. To me, ultimately it seems to be an emotional decision and again you are not really in control of that. Some will have no problem not going and not feel particularly bad about it while others will.
So again, I don't think it is possible to just take an event and look at it completely isolated like you say "Somebody just decide to plough through pedestrians"
as if they were just driving down the road and out of the blue decide to do it. I think that is to oversimplify things to the point where it doesn't make sense, unless the person is physically insane, which would then be the explanation for their action.