In his perush on the bracha "Magen Avraham" (The protector of Avraham), the Alter Rebbe Zt"l has a tremendous chidush.
He asks why in the world Avraham needs a Magen? Rashi on the brit bein habetarim (בראשית ט"ו:א) brings the midrash (ב"ר מד:ה, פדר"א פכ"ז) to explain that Hashem is reassuring Avraham. Avraham was worried that he would be punished for the souls he killed in the war against the 4 kings, so Hashem is promising to "protect" (Magen) Avraham from punishment and promises him great reward. The question still remains, however, why this should be the way we end the bracha of the avot, the very beginning of our prayer to Hashem?
The Alter Rebbe answers that a person need protection from the extreme type of chesed that Avraham exemplified. More than that, the whole world need protection from such chesed that transcends the normal boundaries of balanced middot since it does not even have the slightest component of Din. With chesed alone, we are incapable of fulfilling our task in the world: 'לתקן עולם במלכות שד-י' (to perfect the world through the sovereignty of G-d).
In the words of the Alter Rebbe (תורה אור - לך לך):
דהנה אברהם הוא בחינת חסד גדול עד מאד וכולם טובים בעיניו והוא רק טוב ולכן אמר 'לו ישמעאל יחיה לפניך' - אבל באמת אין מהראוי לעשות כן. דהנה בביאת המשיח צריך לבער כל הרע מן העולם ואם יתנהג העולם בחסדו הגדול של אברהם לא היה העולם יכולים לבא למדרגה של משיח'Since Avraham is the archetype of tremendously great Chesed and all people are good in his eyes and he himself is all "good." This is why he says "O that Ishmael might live before you!" (בראשית י"ז:י"ח) [Avraham concentrates on the good in Ishmael exclusively]. But in truth, such behavior is not proper. Because with the coming of Mashiach there is a need to uproot all evil from the world and if the world will behave in accordance with the great Chesed of Avraham, it would be incapable of reaching the lofty level of Mashiach (the messianic age).
