Freedom from the Yetzer Harah and the Satan
ויðרשע הקהה את שי The night of the Seder is known as עת הזמיר the time of severing Am Yisrael from the galus (the word זמר can be translated as found in the passuk כרמך לא תזמור pruning). At the time of Hashem's revelation in Mitzrayim we are freed from the Yetzer Harah and the Satan. That is why we don't find any kitrug or prosecution against saving Am Yisrael while killing the Egyptian first born. It is only at Kerias Yam Suf that we find such an opposition and prosecuting complaint from above.
This is hinted in the word זמר which the next letter in each of them spell נחש. The שטן (yetzer harah) is an accomplice with the נחש as Chazal mention in the chait of the Aitz Hadaas (Shlah Hakadosh Toldos writes they are the same creation). They both have in their names the letters of שן which are the holy letters that give them power and existence. The letter ח of שחנ and ט of ןטש are the tumah letters that cause one to do a חט an aveirah. The night of the Seder which was full of Hashem's light, removed the letters of חט leaving just the kedusha of ןש which is the gematriah of הזמיר הגיע to remove from Man (Yisrael) the Satan and Nachash. The light of Hashem was so great that the חט of them both were momentarily elevated to חט or חטה – matzah from wheat to be eaten at the Seder. It is the Nachash and the Satan Yetzer Harah that clouds up our emunah in Hashem.
However, at the night of the Seder the mitzvah of matzah is called by the Zohar מיכלא דימהמנותא bread of emunah for the two faceted instigator have been subdued and their חט of evil have been channeled into gematriah ט בו . The Nachash associated with the letters of זמיר who strives to cut you off from Hashem has himself been cut off from you.
