The Sefer Likutei Yehuda cites from the Siach Sarfei Kodesh what the Chidushei Harim commented on the pasuk, in the discussion of Shechem and Shimon and Levi, ויקחו שני בני יעקב...חרבו, the two sons of Yaakov, Shimon and Levi, Dina’s brothers, each took his sword.
The Chidushei Harim explained why the people of Shechem were liable to death. The halacha is that an Eino-Yehudi, i.e., idolater, that observes Shabbos is liable to death. This is because Shabbos is an אות, sign only for the Jewish people. An Eino-Yehudi who uses this sign is considered a stealer and a Ben-Noach receives the death penalty for stealing. Accordingly, the same would apply to Bris Mila which is also called a sign. If an Eino-Yehudi circumcises himself not for the sake of converting he is liable to death.
The children of Yaakov said אך בזאת...כל זכר, only on this condition will we acquiesce to you: If you become like us by letting every male among you become circumcised — meaning to circumcise for the sake of conversion and become full-fledged Jews. Now, concerning a ger we apply the concept of גיטו וידו באין כאחד, the get and the hand arrive simultaneously, and therefore they can circumcise themselves. However, the people of Shechem circumcised themselves for their desires but they wanted to remain as Eino-Yehudim. Therefore, they were liable to death for stealing the sign of the Jewish people.
The Chidushei Harim remarked that Shimon and Levi were 13 at the time and this chidush was their bar mitzva drasha they said.
