> If your brain knows you don't have free will and I don't have free will, it will not fault me for not having a thing I am supposed to not be having.

Punishment decreases the odds of repeated offence. It doesn't matter if it's because you have free will, or simply because you learnt not to do that because your internal neural network were taught not to do that or it hurts.

I'm pretty sure I can make a simulation with agents punishing each other. Doesn't mean they have a free will.

> You can't have the notion of blame if you don't have a concept of free will

I have a notion of free will. I just don't believe it's true. I can model beliefs of Muslims without believing in Allah.

> Same way you can't express certain colors, if you lack the words for it.

Bad example. I can express any color I can see with 3 real numbers. There's infinitely many of them and my vocabulary is finite.

> Fun fact, when people read about not having free will, they are more likely to lie/cheat on a test.

And when they drink they are more likely to do stuff they later regret. Environment influences the brain in many ways. Doesn't mean there is a free will.

> Punishment decreases the odds of repeated offence.

So does running away. However, the issue here is existence of blame.

> I can model beliefs of Muslims without believing in Allah.

Yeah, but you can't model belief without knowing what believing in something entails. Same thing with my example. You model behavior of others based on your experience. You think others have free will because you believe you do. Or not.

My own position is that while free will might or might not be illusion, it's a useful one. Like justice, mercy or love - that sort of thing.