that's the oldest question, isn't it?
i would argue that God can control the evil in the heart, but that He loves us enough to let us choose. He is a gentleman. He doesn't force us to do anything. He doesn't want robots; what satisfaction is there in a bunch of beings worshiping you cuz you forced them to? How much better if they come to that of their own free will? thus, free will. we choose one way or another, all day long. It's the greatest gift.
What we do with that gift is what defines us.