The holy Alshich mentions that it’s a big Kashya (question) how can we atone for our worshipping a: טְלֶה sheep all these years, just like the Mitzri'im worshipped it? He says that after using this: טְלֶה sheep for a Mitzva of Pesach, our Aveira became a Mitzva, and our: קַטֵיגור prosecutor became a: סַנֵיגור defending Malach. This is the same principle of Teshuva, where the evil enemy Malach you created becomes your best friend, and he sticks up for you instead of hurting you.
He asks why were we notified about Korban Pesach on Rosh Chodesh Nissan (10 days before), and he answers that to get rid of the 10 בְחִינות of Tum'ah which were created by our worshiping sheep, we need 10 days from Rosh Chodesh Nissan till 10 Nissan to have a Mach’shava Tova of wanting to bring a Korban Pesach, and the power of that Mach’shava Tova destroys 10 בְחִינות levels of Tum'ah. Look how much power there is in a Ratzon Tov and Mach’shava Tova!
And look how we start off being a Yid with the unreal Chesed that only a Baal Teshuva has, that his very sin becomes the biggest Mitzva. Now, our Korban Pesach tastes even more delicious, when we see how our very: חֵטְא sin becomes our biggest Mitzva!
The Nesivos says that the reason for the Mitzva of Seudas Purim is since they did such a horrific: חֵטְא sin of: שֶנֶהֱנוּ מִסְעוּדָתו שֶל אותו רָשָע מגילה יב א having pleasure from the Seuda of that Rasha, and then they did Teshuva, so their Seuda became a Mitzva, and therefore we need to have Seudas Purim. Notice how both Purim and Pesach our main festivity is our former: חֵטְא sin-turned-Mitzva! How much Chizuk we get when we see the power of our mere Mach'shavos Tovos and our Ratzon Tov, and how the evil in our lives gets converted to major goodness.
We also get a special Mitzva and a super Zechus of fearlessly Shechting the g-d of the Mitzri'im, and eating it before the Mitzri'im’s eyes. We fear only Hashem.
