Assuming that the word 'god' refers to the 'creator of everything' and is 'perfect' (the majority of definitions) then the existence of 'free will' (if it does, indeed, exist) precludes the existence of such a god.
If I have free will, it stands to reason that I am able to apply it to creating something that did not exist before as a direct result of the unfettered choices I have made within the non-confines of that free will (In other words, a god would not have had any control of those choices and, therefore, also have no authorship of the creative result.)
Therefore, I would have created something that was not created by god.
Therefore, everything would not have been created by god.
Therefore, god would not exist as defined, i.e. creator of everything.
Therefore, god can only exist if there is no free will.
Therefore, if there were a god, there could be no sin (imperfection), since everything would be his doing.
This is a fascinating discussion of 'free will' - but much misses the irony of my point - in that it poses an argument in favor of atheism based on a standard argument used by Abrahamic believers - that god created everything - except he didn't. Actually, they are stuck with a tremendous paradox we, as atheists, are not. Because, if god created free will - he made it possible and, therefore, certain, that some things are NOT of his creation - but if he didn't create free will, then he is pre-damning many people to hell (which JW's and Calvinists sort of agree on.)
Of course, as an atheist, if there is no real random number (a no-cause cause), we also have a problem because that suggests (albeit on a chaos theory level) that everything is still pretermined by a hyper-complex multi-chain of causality and, in a way, suggests a 'Master Plan' of sorts (albeit it without an actual 'Master' behind it.).