Chesed precedes and tempers judgment

המכסה אני מאברהם כו' כי הבורא יתברך קודם שעושה דין הוא מתלבש בהחסד והטעם כי עולם חסד יבנה והוא קיום העולם ז"ש ואברהם היה יהיה וכו' ונברכו בו כו'.

"Shall I conceal from Avraham..." — for the Creator, before He carries out judgment, clothes Himself in chesed (kindness). And the reason is that "the world is built on chesed," and it is the sustaining force of the world. This is the meaning of "and Avraham shall surely become..." and "and through him shall be blessed..."

Before executing din (judgment), Hashem first enrobes Himself in chesed, because the world is founded and held in existence by kindness (Tehillim 89:3). That is why, on the verge of judging Sedom, Hashem speaks of Avraham — the embodiment of chesed — through whom all nations are blessed (Bereishis 18:17-18).

וע"י מדת החסד של אברהם אע"ה נמשך ברכה לכל הברואים ע"כ לא יעזוב את חסידיו חסידו כתיב.

And through the attribute of chesed of Avraham our father, peace be upon him, blessing is drawn down to all created beings. Therefore "He will not abandon His chasidim" — yet it is written "His chasid" (in the singular).

Avraham's middah of chesed channels blessing to all of creation. The pasuk (Tehillim 37:28) is written with the singular "chasido," hinting that this flow comes through one singular tzaddik — Avraham — the wellspring of kindness for the world.

וכמו כן הוא נמלך בכל הג' אבות לכן נקראו אבות כי אין דבר נעשה בלי ידיעתם.

And likewise He consults, as it were, with all three Avos; therefore they are called "Avos" (fathers), for nothing is done without their knowledge.

Just as Hashem "consulted" Avraham before Sedom, so He involves all three Avos; they are called "fathers" precisely because they are the roots through which everything passes, so that nothing happens in the world without their awareness.

וראינו כי לא נעשה משפט בסדום עד שנמלך בו ומיד התחיל מדת החסד להרבות בקשה כמה פעמים עד שהסכים ומכל שכן עתה שגדולים צדיקים במיתתם יותר מבחייהם:

And we see that judgment was not carried out upon Sedom until He consulted with him, and immediately the attribute of chesed began to multiply requests many times over, until he assented — and how much more so now, since "the righteous are greater in their death than in their life."

Sedom was not judged until Avraham, the man of chesed, was consulted, and his kindness moved him to plead repeatedly. If his chesed had such power in his lifetime, all the more so now, for tzaddikim are even greater after death (Chullin 7b) — their attribute of chesed continues to draw blessing and shield the world.

Summary: Before bringing judgment, Hashem first clothes Himself in chesed, for the world is built and sustained by kindness — which is why He turned to Avraham, the embodiment of chesed, before judging Sedom. Avraham's kindness draws blessing to all creation, and Hashem involves all three Avos in His conduct of the world, so that nothing happens without them. And since tzaddikim are even greater in death than in life, the protective power of Avraham's chesed endures all the more.