Because he was fated to choose in a specific way. His mind, his brain, his body would act in a very specific way when placed within that circumstance. The causal chain that placed him there also set beforehand the exact outcome of that moment. He would feel and think one specific set of things and thus choose one specific alternative: killing.
That one doesn't seem to have much to do with the idea that a person is controlled by what he wants and so is determined by what he wants.
But I guess you could link the causal chains to what a person wants at a particular time, and even take into consideration that butterfly flapping it's wings in the Amazon as part of the causal chains that link together.
So our reasoning is no more than a justification for what we want, or is our reasoning and feelings the cause of what we want.
If reasoning is no more than justification for what we want then we cannot trust our reasoning ability.
If reasoning is a cause of what we want then it is no longer our wants but our reasoning (and of course our feelings) which cause us to do things.
Whichever it might be, this fate comes back to the cause/effect line that began with the big bang or before.
A tiny change there would change everything I guess. All very interesting to consider but of course the presumption is that humans are no more than the physical and that our reasoning and feelings and wants are all determined through the line of cause/effect in the material world.
If there is a spiritual soul involved which is not directly a part of that material chain of events then the whole thing falls apart.
So if there is any truth to your argument it is solely a materialist argument and presumes materialism imo.
The argument sort of boils down to, because all is material and material is governed by cause/effect chains then all that happens is pre set, determined in advance.
This presumption of materialism seems to be also in the idea that foreknowledge of what we will do and what will happen can only happen in a deterministic universe. It discards, at the start, the spiritual, God, and the possibilities that exist with that outside the cause/effect materialist possibilities.
To a materialist the only way God can know is if there is determinism and a materialistic cause /effect universe.
The argument usually works from God's foreknowledge to a conclusion that says we cannot do anything other than what God knows, so we are not free to choose. The thing that locks us in a prison in this argument is the foreknowledge of God, as if that foreknowledge is forcing us to act a certain way.
But of course someone knowing what we will choose is not a force that makes us act a certain way. This argument ignores the omnipotence of God and that He somehow actually can know what we will freely choose and can foreknow what will happen randomly.
I hope I am making some sense to you.
