What made Avraham worry about Sarah's beauty at this point?
Rashi: Avraham was worried about his wife's beauty, because the Egyptians (sons of Cham) were black and ugly, and were not used to seeing beautiful women. 1
Seforno: Egypt was known to be a haven of promiscuity. 2
Ramban: Avraham was only worried when they approached the capital, the seat of the king's palace, 3 since it was customary to take beautiful women captive on behalf of the king, and to kill their husbands on some preprint or other.
Bearing in mind that Cana'an too was the son of Cham, why did Avraham not worry from the time he entered Eretz Cana'an? This is not difficult, because he only became aware of Sarah's beauty on his way to Egypt. What is difficult however, is why Rashi says that he Egyptiane were back, when it was Cana'an (Cham's youngest son) whom No'ach cursed, and not Mitzrayim (his second son)? See 9:28.
Indeed, says the Ramban, it was Avraham's custom to refer to Sarah as his sister (under the same circumstances) from the time they left Charan, but the Pasuk only mentions when something out of the ordinary occurred.
What would the Egyptians gain through killing Avraham (to avoid transgressing adultery), seeing as murder was just as much forbidden to them?
Rosh, Riva #1, Moshav Zekenim: It is better to transgress murder once, than to transgress adultery on an ongoing basis. 1
Riva #2: The world had already been punished [through a flood] for immorality, but not for murder.
Da'as Zekenim (to 12:11): If they did not kill Avraham, perhaps he would complain to the king.
Peninim mi'Shulchan ha'Gra: If a Choleh must eat meat on Shabbos, if needed, we slaughter for him; we do not tell him to eat Neveilah, for then he transgresses for every k'Zayis (Ran). This is like we find here.