Everything that exists is because of random particles bumping into each other in different ways. If there was a predictable pattern of sub-atomic behavior or a destination for each quark in the universe then we could rule out the idea of free will. But like you said, I don't know if it's that simple. Our genes pre-dispose us towards certain behaviors, but it's not like every part of our being is programmed as if we were robots. I think we have some free will but there is still a lot outside of our control. We can't control getting a disease or having a certain sexuality. I think if life was pre-programmed by a higher being, then life would be the bare bones of a code made entirely from atoms, and free will would occupy the rest of the code. It may sound crazy at first, but not all things in life are clear-cut, black and white. Free will and pre-determinism are two contradicting ideas, but that doesn't mean that one has to be false in order for the other to be true. That's just my take on it.
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