Genesis 20:1-4 meaning

Occasionally pasture land becomes depleted and one has to move herds to better grazing land. This is possibly why Abraham journeyed from there toward the land of the Negev. Negev means "south" and denotes the region south of the Jordan Valley area and south of the Dead Sea. Abraham had passed through the Negev on his way to Egypt during the time of the famine (Genesis 12:9, 13:1-3). For an unspecified period of time, Abraham pastures his herds in the region settled between Kadesh and Shur, cities located in the Negev. This was near Beer-lahai-roi, where the angel had met Hagar (Genesis 16:7, 14). 

At some point, Abraham traveled north and sojourned in Gerar. Gerar was 12 miles south of Gaza (Genesis 10:19) and 50 miles southwest of Hebron (modern Tell Abu Hureirah).

Abraham was afraid that the king of Gerar would kill him in order to take Sara as his wife (Genesis 12:11-12) so Abraham said of Sarah his wife, "She is my sister." Apparently, wife stealing was a common threat. The  word "Abimelech" is likely a royal title (like the Egyptian Pharaoh or Roman Caesar) and not a personal name (Genesis 26:1, 8, 16, 26, 37:36; Psalm 34; Exodus 1:15; 1 Samuel 21:10-15). Although the passage does not presume that knowledge, and tells us Abimelech is the king of Gerar. Thinking Sarah was Abraham's sister, Abimelech king of Gerar sent and took Sarah. It is possible that the king recognized the wealth and military strength of Abraham (Genesis 13:2, 14:14), therefore he wanted to establish a relationship or alliance by marrying his sister for prestige and economic advantage (Genesis 21:22-32).

But God came to Abimelech in a dream. Abimelech had taken another man's wife unknowingly. We do not know the time between when Sarah was taken, and the dream occurred. It should be noted that dreams are often used to provide divine revelation (Genesis 28:12, 31:10-11, 37:5-9, 40:5-8, 41:1; Numbers 12:6; Judges 7:13; 1 Kings 3:5; Daniel 2:3, 4:5, 7:1). God's message to Abimelech is quite direct. God tells him: Behold, you are a dead man because of the woman whom you have taken, for she is married. There is no nuance in this message.

Abimelech defends his innocence. The text confirms that he had not come near her, meaning he had not approached Sarah for sexual relations. The reason Abimelech abstained will be revealed later (vs. 6,17-18). Appealing to God's justice, he asks, "Lord, will You slay a nation, even though blameless?" He pleads that he did not know Sarah was married. 

Abimelech states that if God were to bring about his death, that He would slay a nation. Presumably this is an assertion that his people cannot live without him. This inflated view of himself fits with his apparent desire to expand his harem by adding Sarah. 

Here the word "nation" in Hebrew is goy and is usually translated into English as nation or country. However, sometimes it can refer to a group of people rather than to a nation, and can refer to all Gentiles (Genesis 10:5; 2 Kings 6:18; Psalm 43:1; Isaiah 26:2).