Rashi cites the Mechilta which takes note of the Torah’s emphasis on the staff “with which you struck the river.” When Bnei Yisrael were clamoring for water, Hashem said to Moshe, “Take the same staff that you used to bring plagues upon Egypt and strike the rock with it, and water shall come out.” Let Bnei Yisrael see that the same staff which brought about plague can also bring good. There is no dichotomy between the staff that brings evil and the one that brings good. Hashem is the source of both, and from Him only good emanates. We do not always perceive the good within the “bad.” We do not always perceive that out of sorrow and suffering emerges joy and happiness. It is necessary to have faith that the staff of evil will effect good. Indeed, the good is there — we have only to “see” it.
There is yet another approach to understanding the words of Chazal. Every situation/occurrence which we experience, be it sad and tragic or festive and joyful, comes from Hashem for a specific reason. Consequently, we must recognize the good, as well as the bad. Chazal teach us not to attribute the bad as coming from some other source. No, the good and the bad are both present to serve a purpose — to sanctify Hashem’s Name and to inspire us to take note.
In referring to Tishah B’Av, our day of sorrow, Yermiah Ha’Navi calls it a “mo’ed,” a word which is usually used in reference to a festival. This seeming inconsistency is explained by Horav Avraham Yitzchak Bloch z.l., in the following manner. The word mo’ed, sgun, is derived from the word vaad, sgu, which means appointment. A moed is thus a time set aside when Hashem has an “appointment” with the world, when His presence and greatness is manifest.
Hashem’s eminence can be realized and seen from two perspectives: through the miracles of redemption with the joy and happiness they bring; and through destruction with its pain and sorrow. The glorious exodus from Egypt inspired a nation to perceive gadlus Ha’Boreh, the greatness of Hashem. Likewise, the devastating churban, destruction of the Bais Ha’Mikdash with the ensuing slaughter of Jewish life, was such a catastrophic tragedy, that it could have occurred only as part of a Divine plan. These two distinct empirical moments in history, geulah, redemption, and churban, destruction, were both appointments with Hashem, when His Presence was uniquely manifest. The staff of “evil” and the staff of good are one and the same. We have only to delve into its message.