Assuming that there exists an omnipotent, omniscient, omnipresent being, I find it very difficult to imagine that this being would be purely benevolent. All that is considered good would be his work, but so would all that is evil. The premise presented in the OP doesn't prove that a god could not be omnipotent, but that a god could not be both omnipotent and purely benevolent. An all-powerful being would probably not view good as we define it as being inherently better than evil, but as equally necessary to the function of the universe.
I also find it hard to believe that a god could at once give humans free will and have planned out all that has and ever will be. Either could be true, but I don't see the two as reconcilable with each other.
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