ציטירנדיקער פּירוש אויף בראשית 20:11

Similarly, when challenged by Avimelech about his plot to conceal his wife’s true identity, Yitzchak is basically silent except to say that he was scared for his life (Bereshit 26:9): He saw no need to use this as an opportunity to engage Avimelech in a protracted discussion about ethics. In contrast, Avraham, being much more interested in the spiritual education of others, makes sure to explain that he felt compelled to conceal his wife’s true identity because of the low moral level of the Philistines (Bereshit 20:11–12). Avraham also explains the linguistic justification of calling Sarah his sister to make sure that Avimelech understands Avraham’s own punctilious concern with ethics – even though from a normative halachic perspective, Avraham would certainly have been allowed to lie in order to save his life. Avraham wanted to make a point about Avimelech and his followers’ need to grow religiously. From Yitzchak’s very limited response, however, we see that such a point did not appear to be part of his agenda. Yitzchak’s focus simply did not lay in his interactions with others.

AND YET SHE IS MY SISTER, THE DAUGHTER OF MY FATHER. I know not the sense of this apology. Even if it were true that she was his sister and his wife, nevertheless when they wanted to take her as a wife and he told them, She is my sister,²⁷⁸Verse 2 here. in order to lead them astray, he already committed a sin towards them by bringing upon them a great sin,²⁷⁹Verse 9 here. and it no longer mattered at all whether the thing was true or false!
Perhaps it was because Abimelech said, What sawest thou, that thou hast done this thing?²⁸⁰Verse 10 here. meaning “What sin or wickedness have you seen in me that caused you to do this out of fear? I have never attempted to take women away from their husbands.” Then Abraham answered: “I did not know you, but I thought, perhaps the fear of G-d is not in this place,²⁸¹Verse 11 here. for in most places in the world there is no fear of G-d. Therefore, from the time I left my country and aimlessly wandered among the nations not knowing to which place we would come, I made this condition with her that she say thus in all places. For the matter is true, and I thought that by doing this, human life would be saved. I did not begin this practice upon entering your country for I did not see that you had committed any sin.” And yet she is my sister, the daughter of my father. This expression represents a different argument. Abraham said: “According to my custom, I said so for it is true, and I further thought that in case they will want her they will ask me if she is also my wife. Since your servants took her and they did not ask me any questions, I said, ‘The fear of G-d is also not in this place’ and I remained quiet.”
It is possible that he established this condition with her when they came to Egypt even though he said that he did it when G-d caused me to wander from my father’s house.²⁸²Verse 13 here. It may be that he again warned her there in Egypt at the time of the event, as I have explained.²⁸³Above, 12:11.
Rabbi Abraham ibn Ezra’s opinion is that all these words were to put off Abimelech.

The Torah wants us to understand that Amalek’s pirate-like existence did not emerge as a fluke, but rather was deeply rooted in their approach to life more generally, i.e., in what the Torah calls “not fearing God.” On the face of it this is a strange claim, for which pagan nations of the time could actually be described as God-fearing? And yet Amalek is the only one the Torah singles out as not fearing God. It is important to note that from among the few times the concept is mentioned in this form is when Avraham says Avimelech’s country was devoid of the fear of God (Bereshit 20:11) and when Yosef, posing as an Egyptian, reassures his brothers that he fears God (Bereshit 42:18). As these references are dealing with non-Jews, it appears that even if almost all of the nations of the time were idolatrous, there was nevertheless an expectation that they should be – and, in fact, often were – “God-fearing.”