What caused Avraham to state that there was no Yir'as Hashem in the town and that they would kill him on account of his wife?
Seforno: Avraham was referring (not to Yir'as Hashem, but) to fear of the realm, which leads to anarchy. His fear was based on the fact that the 'Sarnei (the rulers of) P'lishtim' were not real kings. 3
Ramban (12) #1: He was not actually accusing Avimelech of a lack of Yir'as Shamayim. He was merely explaining that, since most places lacked Yir'as Hashem, he and Sarah referred to themselves as brother and sister wherever they went, on the assumption that the residents belonged to the majority.
Ramban (12) #2, Seforno (12): He said, if you desired her, you should have asked whether she was not just his sister, but also his wife.
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 Hashem.
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 Shemuel 1, 17:8.