I have basically heard the same reasoning about omnibenevolence vs. omnipotence, and I generally agree with that reasoning. I think we DO have self-determination, which means that we have the potential to do great good and great evil--it's our choice. That also makes us responsible for our own actions. WHY, someone might ask, were we given free choice if we are so prone to misusing it? Wouldn't it have been better to just create a world of people who weren't capable of doing bad things (wouldn't it have been better to, as it has been put, "zap Lucifer"?) Well, yes and no. Sure, the world would be a lot more "pleasant" if we didn't have free choice, and if nothing were cappable of going against God's will. But it wouldn't be a very stisfying happiness. We would be mere automatons, really, and we wouldn't be able to do much. Newton once said that "every action has an equal and opposite reaction". If we human beings are to have the potential to love fully, and to do great good, we must also have the potential (whether it is acted upon or not) to hate and to do great wrong. To really, fully exist, I think we had to have free choice, free will--but by definition that means we must be able to choose whether or not we will follow God's will. That means that God cannot have the power to "control" us (and that's the end of omnipotence). Why any rational person at all would choose to disobay God, and why that potential to do evil as well as good was actualized, I certainly do not know. That is it's very own question outside of the question as to whether we have free will and what that free will might mean.
--Redwing