On the last days of Pesach, we recite only half of Hallel (Psalms 113-8), even though we are celebrating awesome miracles - the splitting of the Red Sea and the end of Egyptian tyranny over Israel.
Why this subdued commemoration of such a liberating event? The Babylonian describes the angels as bursting out in song after the splitting of the Red Sea (Megillah 10b). God, however, rebukes them and says, "My handiwork is drowning in the sea, and you are singing?"
The angels' attempt at praise pained God, as it were. God expected the angels to understand that, at that moment, joy was tinged with sadness .
The message of the half-Hallel is that when your bitter enemy falls, you may celebrate, but with restraint. We are all creatures of God - even those who wish us ill.