Something that occurred to me over a period of time, that touches on free will:
I have said "If you can convince someone that an invisible man in the sky is going to burn him forever for masturbating, you can convince him of anything".
I also acknowledge that many religious people have the same take with a mirrored perspective: "If you can convince someone that he's just a clever ape that's going to die, you can convince him of anything."
The first is said in an assessment of credulity; that once you have replaced someone's worldview with one that is apparently utterly ridiculous and unsupported by their own experience, that person can no longer exercise judgement.
The second is said in an assessment of morality; that once you have reduced someone's personal worldview to that of being utterly inconsequential, they no longer have any moral compass to dissuade them from committing grievous ills.
I am coming slowly to the realization that both proposals are just the same statement with an unnecessary qualifier added as a prefix. Starkly stated, the truth is:
You can convince people of anything.
If you can manipulate a person's environment, both external (culture, mostly) and internal (drugs, mostly) there is literally nothing that they will not believe. Slavery is good; slavery is bad. Homosexuality is good; homosexuality is bad. Stars are gods; stars are huge balls of nuclear reactions. The world is flat; the world is a globe. The sun moves and the earth is still; the earth spins like a top as it moves round the sun. Killing civilians in war should never be done; killing civilians in war is the best way to conduct war. We're real; we're not real.
Again, where in all this does free will live? Solely in impulse control. What most people call free will is simply a creature reacting to conflicting impulses by choosing the one that most closely mirrors intellectual logic over the one that is driven by a more immediate emotional impulse. That's what we consider free will.
But in deciding to smile at your boss instead of murdering him, one brain function is overriding another part based on a different set of criteria, but the need goal of both brain functions is survival and procreation. Change the criteria necessary to achieve those goals, and the decision changes fluidly. Cement the belief that you will never be caught, and strengthen the belief that you will can forever if only the boss dies, and you can accurately predict the murder.
The exercise of impulse control to rein in reaction to immediate emotional responses. That I believe is the only useful definition of free will.