What caused Avraham to state that there was no Yir'as HaSh-m in the town, and that they would kill him on account of his wife?
Rashi: He made this statement because, 'When a stranger arrives in town, does one ask him 1 about whether his partner is his wife or his sister, 2 or about whether he has somewhere to stay'? 3
Seforno: Avraham was referring (not to Yir'as HaSh-m, but) to fear of the realm, the lack of which leads to anarchy. His fear was based on the fact that the 'Sarnei (the rulers of) Pelishtim' were not real kings. 4
Ramban #2: He was not actually accusing Avimelech of a lack of Yir'as Shamayim. He was merely explaining that, since most places lacked Yir'as HaSh-m, he and Sarah referred to themselves as brother and sister wherever they went, on the assumption that the residents belonged to the majority.
Ramban #2: He was accusing Avimeelch of not asking whether he was not just his sister but also his wife. 5
Gur Aryeh: We know that this was what happened, for how else could Avraham have known immediately that the people did not fear HaSh-m?
The Gemara in Bava Kama (92a) learns from here that, at the end of the day, Avimelech was guilty (and deserved the death-penalty), because he ought to have learned Derech-Eretz and didn't. Presumably, that is what Avraham is referring to here when he spoke of a lack of Yir'as HaSh-m.
The Ba'alei Musar learn from this Pasuk that, however 'nice' a person may be, if he has no Yir'as Shamayim, he is perfectly capable of murder, should the need arise.
As hinted by Golyas, in Shmuel I 17:8.
