Purim's darkness revealed retroactively as light

בפסוק ליהודים היתה אורה כו'.

On the pasuk: "For the Yehudim there was light," etc. (Esther 8:16).

The Sefas Emes opens with the verse describing the salvation of Purim, when the Jews experienced light, gladness, joy, and honor.

יובן עפ"י המדרש תהלים שדורש אור ישראל לאש זה מרדכי וקדושו ללהבה זו אסתר ע"ש סימן כ"ב.

It can be understood according to the Midrash Tehillim, which expounds: "the Light of Yisrael shall become a fire" — this is Mordechai; "and his Holy One a flame" — this is Esther; see there, chapter 22 (cf. Yeshayahu 10:17).

The Midrash reads the pasuk about Yisrael's "fire and flame" as referring to Mordechai and Esther — the fire being a force of consuming destruction toward Yisrael's enemies.

ולפי זה יתכן לפרש היתה על אסתר שעיקר הענין הי' לכלות זרע עמלק והיא להם אש וכליון.

According to this, one may explain "there was" as referring to Esther — for the essential matter was to wipe out the seed of Amalek, and she was for them a fire and annihilation.

The feminine word "haysah" ("there was") hints at Esther, who, like the "flame," was the agent of destruction against Amalek, whose eradication was the deepest purpose of the Purim story.

ולבני ישראל היתה אורה ושמחה.

And for Bnei Yisrael she was light and joy.

The very same Esther who was fire and annihilation to Amalek was, for Bnei Yisrael, a source of light and gladness — the two faces of a single event.

וכולל ג"כ כל המעשה שהי' הסתר וחושך ואח"כ נתהפך מאבל לשמחה.

And it also includes the entire episode, which was concealment and darkness, and afterward was transformed from mourning to joy.

The word "orah" (light) encompasses the whole Purim narrative, which began in hester (hiddenness) and darkness and was then overturned — "v'nahafoch hu" — from grief into rejoicing.

ונתברר כי כל המעשה והגזירה ליהודים היתה אורה.

And it became clarified that the entire episode and the decree was, for the Yehudim, light.

In hindsight it became clear that even the terrible decree of Haman was, for the Jews, ultimately a source of light and good.

וממילא משתמע רק ליהודים אבל להם היה חושך וצרה ולכן כתיב היתה שנודע למפרע שהי' לטוב לישראל:

And it follows by implication "specifically for the Yehudim," but for them [the enemies] it was darkness and distress — therefore it is written "there was," for it became known retroactively that it had been for the good of Yisrael.

The phrasing "for the Yehudim there was light" implies that for their enemies the very same events were darkness; and the past-tense "haysah" teaches that only afterward, looking back (l'mafrei'a), did it become clear that the entire ordeal had all along been for Yisrael's good.

Summary: The pasuk "for the Yehudim there was light" carries a double meaning: Esther was a consuming fire of annihilation to Amalek yet light and joy to Bnei Yisrael. The whole Purim story began in concealment and darkness and was overturned to joy, teaching that even Haman's fearsome decree was, in retrospect, entirely for the good of Yisrael — a good that only became apparent l'mafrei'a, after the fact.