Why go through the incredibly painstaking process of incarnating himself as a human in a world crawling with self-professed prophets and then sacrificing himself?
Because some of us so stubbornly cling to our imperfections as something which bars us from communion with the Divine that it took such an Act to convince some of us of our error.
‘Free will’ was advanced forward in order to try and explain away the inconvenient paradoxes within a theistic universe. If humans can voluntarily choose to be evil (which makes us worse than god because he isn’t) because we have such a thing as a ‘free will’, then it no longer becomes a problem with god but with mankind. The problem is that free will doesn’t make sense.
You can theorise a soul as a sort of spiritual facsimile of your conscious being that isn’t subject to entropic demise, however the fleshy bit of you; the body and the brain and the resulting consciousness are subject to causality. No human being has exactly the same cognitive and physical abilities as the next person and they partially determine the course of action that you take and the decisions that you make. Therefore no human is ‘fully informed’ of all choices that are available. You are afforded the choices that evolution has granted you and what causality has determined. There is a chunk of the human experience that as someone with very mild Aspergers is forever just out of reach. Given that spiritually speaking, that may result in me by default making a ‘sinful’ decision that someone else may exercise their volition and avoid then one would hope that wouldn’t be held against me.
But it would be held against me, if you keep advancing the idea of ‘free will’; it’s why so many countless people have been told when facing an experience that to them makes perfect complete and utter sense but runs contrary to doctrine that you’re just not ‘trying hard enough’ to see things the way they ought to be. Even if you were to assume that the soul was the ‘fully informed’ part of you then it can be easily inferred from the actions that people take, that this soul is incapable of informing the rest of you, because you’re still making ‘mistakes’ as your cognitive abilities are still subject to your consciousness which is still rooted in the physical.
If the concept of a genuinely free will isn’t compatible with a materialistic view of the universe, even if you subscribe to a spiritual/metaphysical view of the universe (where there is a soul), there isn’t any evidence that this soul is informing you in addition to or in place of your consciousness.
If the soul was truly ‘free’ from that ( yet still somehow ‘you’), it would allow you to experience cognitive processes that you would not otherwise be able to experience due to your physical limitations. If it was making its presence known then it should, at least occasionally be able to ‘burst out’ of your physical and cognitive confines rather than hide behind it. Yet this doesn’t happen. If the soul is acting behind the scenes, then it’s following exactly the same processes as your body and isn’t guiding you any more or any less than your consciousness is in making moral choices.
So how can you say that a soul exists independently or even co-dependently of your consciousness? How can you say that it exists at all? But it has to exist in order for ‘free will’ to exist in turn in order to help excuse god’s theodicy. So much has to be constructed in order to excuse the acts of god that manifest themselves in the Bible.
Your devotion to the idea that order equals "good" betrays you, as it does many others, both theist and atheist. That human nature includes chaos (i.e. free will) doesn't make us inferior to a being of greater order, it makes us different. We tend to think that difference makes the Divine incapable of cherishing us, but like many assumptions concerning the unlovableness of that which is different, it is incorrect.
I also find error in your assumption that failure to do something which you are incapable of doing somehow equates to evil. The ability to be omnibenevolent does not require omnipotence, only omniscience. Plus, keep in mind that when it comes to philosophy/theology, the operative definition of 'omni-' is not 'all things imaginable' but 'all things possible' since the former leads to paradox.
I also fail to see why an immortal soul is required for the existence of free will. Immortality is inherently orderly while free will is chaotic. The two can coexist, but neither is dependent upon the other.