The Talmud in Masechet Megilla, elaborates on the well known Midrash regarding the demise of the Egyptians.
Hashem reprimands the angels for singing praises to Him. He tells the angels, “The drowning Egyptians are the work of My hand, and you want to sing Me songs!”
The hint to this Midrash comes from the Pasuk that says that during the evening before the splitting of the Red Sea, they did not come near to one another that entire night. The Hebrew for not coming near one to another is, זה אל זה. The more familiar usage of these words, is found in the daily Kedusha prayer, when we recite that we need to sanctify Hashem in this world, as the angels sanctify Him in the Heavens, when, וקרא זה אל זה, when they call out to one another, “קדוש, קדוש, קדוש.”
The connection between the similar usage of these words, is the basis for the reprimand of Hashem to the angels. And this is also the reason why we only say a full Hallel on the first day of Pesach.
The seventh day of Passover, is the actual day the Egyptians were drowned, and because it is a holier day than the Intermediate days of Chol Hamoed, we treat the last six days the same, without the recitation of Hallel.
This Midrash does not mean that we are not allowed to rejoice when our enemies fall. We pray that we will see this rejoicing very soon, when Hamas will be totally defeated!