One of the most intriguing elements of Jewish history is that when Jews have had the opportunity to take revenge on our enemies, usually we're not so interested. There's a big Holocaust literature on it. Generally, Jews have had a capacity for moving on and building better lives. As for revenge, better to leave it to God, who does a far better job and has far better resources. This particular verse comes exactly where the focus of the seder switches from the past, the history of the Jewish people, to the messianic future described in the second half of the Hallel. The Shulchan Aruch says that you open the door to show that this is a leyl shimurim, a night of divine protection, and that God will now address the problem of "those nations who refuse to know You." It's basic to Judaism to believe that once free will is given to mankind, there will be both individuals and societies that make use of that free will in a negative manner, and that before there can be complete messianic redemption, some will offer a very ungodly resistance. The prayer here is nothing more than saying, "God, clear the path for the final, universal recognition of who You are."
Rabbi Yitzchok Adlerstein
Los Angeles, CA