What is the significance of the word "Vayashkifu" (as opposed to "Vayir'u" or "Vayabit"?
Rashi, Hadar Zekenim, Moshav Zekenim and Seforno: 'Hashkafah' always has negative connotations. 1
Seforno: The Seforno adds that it was because they were the antithesis of Avraham, who was the very embodiment of Chesed. 2
Since when do angels need to be accompanied?
Rashi and Seforno: Avraham still believed them to be men. 1
What is the significance of the last phrase in the Pasuk?
Seforno: It was because Avraham was performing the Mitzvah of accompanying his guests that Hashem informed him of His intentions to destroy Sedom, so that he could perform another Mitzvah (of Chesed). 1
Moshav Zekenim: Because Avraham himself accompanied his guests, Hashem Himself led Bnei Yisrael in the Midbar.
In keeping with the Mishnah in Pirkei Avos (4:2) 'One Mitzvah leads to another. It also demonstrates the difference between Avraham (who typically was busy with an act of Chesed, whilst Sedom was the very antithesis of Chesed. Refer to 18:16:1:1. Moreover, it highlights the greatness of Avraham Avinu, who Davened for people who were not only wicked, but who were the very antithesis of all that he stood for.
Which angels departed?
Rashbam and Targum Yonasan: The two 'junior' angels (Refa'el and Gavriel) now left for Sedom; Micha'el (the senior of the three) was still talking with Avraham. 1
Presumably, he is the angel conveying Hashem's words to Avraham up to Shelishi. According to Targum Yonasan, he then ascended back to Heaven.
רש"י: כל השקפה שבמקרא לרעה: למה 'השקפה' היא לרעה?
גור אריה: 'השקפה' מלשון מכה 1 , ו'השקפה' היא לרעה כי היא מכה בכח על הדבר.
כמו (לקמן מ"א,ו') "שקופות קדים".
רש"י: ללוותם, כסבור אורחים הם: ודאי סבר שהם אורחים שהרי עשה להם סעודה?
גור אריה: היה מקום לומר שאחרי שבישרו לו שתלד שרה נודע לו שהם מלאכים, שאם לא כן- מנין להם דבר זה? ולכן השמיענו רש"י שעדין כאורחים נידמו לו.