Or HaChaim on Genesis 20:6/אור החיים על בראשית כ׳:ו׳

1 ויאמר אליו האלוקים. G'd said to him (Avimelech). He agreed that Avimelech had not yet become guilty of death considering he had been convinced that Sarah was not married. As to his integrity, however, this was another matter. Whereas he might not have literally "robbed" Sarah [i.e. she did not belong to Abraham. Ed.], nonetheless he had employed the authority of being the king which is a kind of violence. At the very least "his hands were not clean in the matter."

2ואחשוך גם אנכי אותך, "I too saved you from sinning." G'd made it plain that when He said that Avimelech was going to die, He meant that if he did not restore Sarah to Abraham but would sleep with her. He had told him of Sarah's status now and thus given him a chance to save himself from sinning.

3מחטוא לי, from committing a sin against Me. Although Avimelech would not have been guilty of death had he slept with Sarah before becoming aware of her status as a married woman, nonetheless he would have sinned against G'd inasmuch as G'd was especially fond of both Abraham and Sarah and any offence against them would have been an offence against G'd Himself. This is why G'd added the word לי, "against Me."

4על כן לא נתתיך, "this is why I did not give you a chance, etc." Because of the special circumstances involved, I prevented you from sleeping with her; had you known she was married you would by now have been guilty of death. G'd judges the Gentiles even for a sin that has been planned but not yet carried out merely due to circumstances beyond the control of the sinner. According to Bereshit Rabbah 52,13 that an angel pushed Avimelech away when he tried to sleep with Sarah, the words: "I did not allow you to sin against me" make perfect sense.