Normally there is not much I can say to change your mind on that, but just note—there are other views, like idealistic monism, which unlock other possibilities wrt. free will, and which cannot be disproven or proven compared to physicalist monism (which cannot be proven or disproven either).
As parent said, many open questions and interpretations become relatively trivial under idealism, while preserving 100% of our scientific understanding and method.
I think the best resource are his books. He also has several online interviews. The guy used to work at CERN and ASML, turned to philosophy.
https://www.essentiafoundation.org/analytic-idealism-course/
A compressed version of the same material
https://www.essentiafoundation.org/ef-keytoe-analytic-ideali...
The PhD dissertation https://www.youtube.com/watch?v=9iyoiJmWvSs
Enjoy the rabbit hole :)
Free will is a matter of definition, and certain definitions, no matter how hard we try, can't be formulated as easily as one does in, say, Euclidean geometry.
So you can come up with a definition of the free will you don't have, let's call it "absolute determinism", and it has all sort of interesting criminal applications, beyond slapping people on the face (yes, who does that anyway?).
For example, you could use your[^1] absolute determinism to build a very advanced AI that relies on a pseudo-random generator with a fixed seed. Fallibility is the result of using stochastic search methods, and your AI shows it. In that respect and many others, your AI acts exactly as a human would because you programmed it that way. Yet, you have the strongest argument possible to affirm that the AI you created has no free-will. One day, the AI leaves a bunch of children mentally dysfunctional[^2]. But it has no free-will, all it is emanates from you. So you must be charged for the AI's crime.
Your lawyers come to court and state that you in turn have no free-will, that you are God's creature. The prosecutor brings a priest to say that God gave you free will. The judge says that precedent demands you be thrown from a high mountain, but it's feeling like a good day for a crucifixion. Of course, the trial is a shamble, a racket run by Sapiens' genes to ensure that they are passed down. And from that point of view, you committed the ultimate sin.
The way out? Submit to your genes, do what they tell you to. Believe in free-will.
[^1] I wash my hands.
[^2] Wait, did that happen already?
That is not true. Your gene expression is influenced by experiences, and not just yours, but your grand-grand parents' experiences, too. See: epigenetics and transgenerational epigenetic inheritance.
> Your lawyers come to court and state that you in turn have no free-will, that you are God's creature.
Some say that lack of free will, i.e. determinism undermines moral responsibility, which in turns means that it conflicts with punishment and so forth. I believe that it ultimately does not matter; you can punish for the behavior alone.
If someone can't "decide" to slap their neighbor in the face and is determined to do that, why can't the same reasoning be applied to society? Society didn't freely choose to lock someone up, it was determined to do so.
That is, I don't have the free will to believe in free will or not.
Hmm, no we can't. And the few people who seemingly can do that, it's because their biology/environment allowed that. So kind of circling back to determinism. Free will is a myth that's used to lock people up.
If you slap someone in the face, we'll lock you up somewhere because we don't know if you'll do it again. It doesn't matter if there is free will or not. Society has decided to put annoying elements in a huge bastille and call it a day.