But God came to Abimelech in a dream of the night, and said to him, “Behold, you are a dead man because of the woman whom you have taken, for she is married.”
Genesis 20:3 (NASB)
The rabbis suggest that the people of Abraham’s day believed that faithfulness is of such great importance that adultery was considered worse than murder.
Back in Genesis 12, Abram’s fear was that the men of Egypt would be willing to MURDER him, but he’s not worried that they’ll simply abduct Sarai and sleep with her or take her as their own wife.
It came about, when he was approaching Egypt, that he said to his wife Sarai, “See now, I know that you are a beautiful woman; and when the Egyptians see you, they will say, ‘This is his wife’; and they will kill me, but they will let you live.
Genesis 12:11-12 (NASB)
Perhaps part of this idea comes from seeing that God commanded faithfulness first in Genesis 2:24, but the story of murder isn’t shown until Genesis 4 when Cain kills Abel.
And perhaps this is why God warns Abimelech the way He does.