Abraham was afraid the people of Gerar would see him as a foreigner and upon seeing his beautiful wife, they would try to kill him. This would free Sarah to marry someone else. Apparently, it would have been easy for them to kill a foreigner without punishment. If she was Abraham's sister, they would have shown favor to him. (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/tw/dict/bible/kt/favor]])
Abraham believed that the people of Gerar did not fear Yahweh, but when confronted by Yahweh, the king showed that he feared Yahweh. Instead, it was Abraham who did not show any fear of Yahweh when he sinned. This is irony. (See: [[rc://en/tw/dict/bible/kt/fear]] and [[rc://en/tw/dict/bible/kt/sin]])