Most theodicies (counter-arguments against the argument from evil) run something like this: "God cannot intervene to prevent evil because if He did so it would go against _______" where we can fill in the blank with something we tend to like, such as free will or personal moral development. Here in chapter 20 we find that the God of Abraham wasn't terribly concerned with these philosophical notions, at least not where Sarah's virtue was on the line. Once again Abraham has played the sister card and given her unto a king (Gerar rather than Egypt this time around) because although she may be ninety years old, she is still smoking hot. God himself warns the king against following in the way of Pharaoh:
"I know that thou didst this in the integrity of thy heart; for I also withheld thee from sinning against me: therefore suffered I thee not to touch her. Now therefore restore the man his wife; for he is a prophet, and he shall pray for thee, and thou shalt live: and if thou restore her not, know thou that thou shalt surely die, thou, and all that are thine."The king is naturally upset with Abraham for pulling the sister stunt, and Abraham excuses himself by noting that the people of Gerar are probably godless killing machines, and anyway Abe's wife really is his half-sister. (Incest prohibitions will come along later in the Torah.)
The moral of the story is somewhat unclear, however, whether it is to be taken as parable or history, it is clear that the God of Abraham has yet to hear of the free-will defence or the soul-building theodicy, since he steps right in to intervene with revelation and miracles and threats of regicide.