Genesis 20:8-11 meaning

The King wasted no time sharing the revelation he had received from God. Abimelech arose early in the morning and called all his servants and told all these things. Abimelech was angry that Abraham's deception had made him guilty of this great sin. He knew that taking a married woman into his harem was a great sin.

Abimelech reprimands Abraham for exposing him and his kingdom to the guilt and punishment of sin by withholding full information about his relationship to Sarah asking, "What have you done to us?" Abraham's actions jeopardized the nation of Gerar. The king challenges Abraham asking, "How have I sinned against you?" asking Abraham what mistreatment deserved retribution. Abimelech points out that Abraham had done to me things that ought not to be done. 

He continues questioning, "What have you encountered, that you have done this thing?" In other words, "What mistreatment have you encountered that would cause you to mistreat us in return?" Adultery was a severe offense and a great sin, so the king wanted to know what Abraham's reason was for doing this.
Abraham answered  that he assumed there was no fear of God in Gerar, so he reasoned that, as a result, he would be killed because of his wife. Abraham doubted there was any respect for one's moral or social rights for human life among these unbelieving people (Psalm 36:1-4, 11; Romans 3:18). The people of Sodom had no respect for a sojourner (Genesis 19:9), perhaps Abraham supposed the people of Gerar would be the same thinking, "they will kill me because of my wife." Abraham's concern was justified, as Abimelech did, indeed, desire and take Sarah. However, Abimelech did listen to God. It is interesting to note that Abraham answers Abimelech's question in a straightforward manner. Perhaps this is part of his prophetic ministry. Abraham will also make it clear that his statement about Sarah being his sister was true.