Genesis 20:3

And a word came from before the Lord unto Abimelek, in a dream of the night, and said to him, Behold, thou diest, because of the woman whom thou hast carried away, and she a man's wife.

Elokim came. An angel appeared to him on Avraham’s behalf. Pharaoh was afflicted with severe plagues (12:17) whereas regarding Avimelech it is written only that Hashem “restrained every womb.” This is because Avimelech was less wicked than Pharaoh.

ויבא אלוקים אל אבימלך, this visitation was of a similar kind to that reported with Lavan (31,24) and Bileam (Numbers 22,24) respectively. The Torah refrains from speaking about a מחזה, “a vision,” in order not to place these wicked people on a pedestal similar to our patriarchs. The expression דבור to describe a verbal communication to these individuals is also absent, G’d having reserved such an expression for communications with prophets, as He pointed out to Miriam and Aaron in Numbers 12,6. Avimelech did not experience a visual image of G’d in his dream at all. He perceived a divine voice addressing him.

הנך מת, the impotence which you experienced (verse 18) will result in your death.

ויבא אלוקים אל אבימלך בחלום הלילה, a similar mode of expression has been chosen by the Torah when reporting G’d as appearing to Lavan and warning him not to harm Yaakov (Genesis 31,24). The wicked who do not deserve a communication from G’d, sometimes experience such a visitation as a mark of honour for the righteous people who are the subject of such a visitation. Pharaoh of Egypt at the time when he had seized Sarah had not been granted even such a dream, but the plagues visited upon him and his servants had been used to indicate the displeasure of Avraham‘s G’d with Pharaoh’s conduct. The differences in G’d’s dealing with different people are described by Elihu in Job 33, 14-16, “for G’d speaks time and again- though man does not perceive it- in a dream at night, a night vision, when deep sleep falls upon men, while they slumber in their beds. Then He opens men’s understanding and by disciplining them leaves His signature.” The two methods G’d employs are either dreams or afflictions.

והיא בעלת בעל, she is out of bounds to you. Even the first human being had already been commanded not to commit adultery, as our sages derived from the words ודבק באשתו, meaning that he is to have intimate relations exclusively with his own wife. (Genesis 2,24) From all we read it is clear that the people of that time considered the sin of adultery so severe that they were prepared to kill the husband in order not to violate the commandment not to commit adultery. (compare the comment made by Avraham to Sarah in 12,12) They were aware of the sin of committing murder but considered it as a lesser sin. Our sages paid attention to the expression בעלת בעל as opposed to אשת איש, suggesting that according to Avimelech’s mores, a woman who had become married but whose marriage had not yet been consummated would not be included in the prohibition of being someone else’s wife. Girls who were merely betrothed would not be considered as out of bounds on account of their being betrothed. [a major difference between Jewish law and the Noachide laws. Ed.]