“…The wicked one, what does he say? “What is this worship to you?” …Say to him: “It is for this that G-d did for me when I left Egypt;” i.e., ‘for me,’ but not for him. If he had been there, he would not have been redeemed…” (The Haggadah)
The wicked son dismissively asks what purpose there is in the observance of the Seder, excluding himself from being among its adherents. Yet instead of answering his question, we respond by telling him that, in fact, he would not have merited to be redeemed from Egypt.
Why not respond to his question? Besides, if he truly has no association with the Seder, then why include this dialogue in the Haggadah at all?
This supports an alternative understanding of our response to the wicked son, according to which the response is intended to draw the wicked son nearer to the Seder, of which even he is a part. We are telling the wicked son that, “If he had been there, he would not have been redeemed,” but in the future Redemption he too will be redeemed.
Only in Egypt, before G-d gave us the Torah, was it possible for a Jew to be permanently detached from the Jewish people, and to not be redeemed together with the rest of Bnei Yisrael. At the Giving of the Torah, however, G-d spoke to the soul of every Jew who will ever live, saying in the direct and singular form, “I am the L-rd, your G-d,” your eternal identity and inescapable destiny. We are therefore certain that ultimately, before the coming of the future Redemption, every Jew will return to G-d and His worship (see Tanya, Chapter 39), and will thereby merit to be redeemed.
With these words of hope, expressing our belief in the unconditional and eternal heritage of every Jew, we welcome the wicked son back to the Seder. For he too must prepare for his part in the future Redemption.
—Likutei Sichos vol. 11, p. 2, Toras Menachem vol. 8, pp. 77-78