רמב"ן על בראשית ל״ד:י״ג:א׳

(יג) ויענו בני יעקב את שכם ואת חמור אביו במרמה הנה חמור ושכם אל אביה ואל אחיה דברו אבל הזקן לא ענה אותם דבר כי בניו ידברו במקומו בענין הזה לכבודו כי בעבור היות הדבר להם לקלון לא ירצו שיפתח פיו לדבר בו כלל ויש כאן שאלה שהדבר נראה כי ברצון אביה ובעצתו ענו כי לפניו היו והוא היודע מענם כי במרמה ידברו ואם כן למה כעס ועוד שלא יתכן שיהיה רצונו להשיא בתו לכנעני אשר טמא אותה והנה כל האחים ענו המענה הזה במרמה ושמעון ולוי לבדם עשו המעשה והאב ארר אפם להם לבדם והתשובה כי המרמה היתה באמרם להמול להם כל זכר כי חשבו שלא יעשו כן בני העיר ואם אולי ישמעו לנשיאם ויהיו כלם נמולים יבואו ביום השלישי בהיותם כואבים ויקחו את בתם מבית שכם וזאת עצת כל האחים וברשות אביהם ושמעון ולוי רצו להנקם מהם והרגו כל אנשי העיר ויתכן שהיה הכעס ליעקב שארר אפם על שהרגו אנשי העיר אשר לא חטאו לו והראוי להם שיהרגו שכם לבדו וזהו מה שאמר הכתוב ויענו בני יעקב את שכם ואת חמור אביו במרמה וידברו אשר טמא את דינה אחותם כי כולם הסכימו לדבר לו במרמה בעבור הנבלה שעשה להם

ורבים ישאלו ואיך עשו בני יעקב הצדיקים המעשה הזה לשפוך דם נקי והרב השיב בספר שופטים (רמב"ם הלכות מלכים פי"ד ה"ט) ואמר שבני נח מצווים על הדינים והוא להושיב דיינין בכל פלך ופלך לדון בשש מצות שלהן ובן נח שעבר על אחת מהן הוא נהרג בסייף ראה אחד שעבר על אחת מהן ולא דנוהו להרגו הרי זה הרואה יהרג בסייף ומפני זה נתחייבו כל בעלי שכם הריגה שהרי שכם גזל והם ראו וידעו ולא דנוהו.

ואין דברים הללו נכונים בעיני שאם כן היה יעקב אבינו חייב להיות קודם וזוכה במיתתם ואם פחד מהם למה כעס על בניו וארר אפם אחר כמה זמנים וענש אותם וחלקם והפיצם והלא הם זכו ועשו מצוה ובטחו באלקים והצילם.

ועל דעתי הדינין שמנו לבני נח בשבע מצות שלהם אינם להושיב דיינין בכל פלך ופלך בלבד אבל צוה אותם בדיני גנבה ואונאה ועושק ושכר שכיר ודיני השומרים ואונס ומפתה ואבות נזיקין וחובל בחבירו ודיני מלוה ולוה ודיני מקח וממכר וכיוצא בהן כענין הדינין שנצטוו ישראל ונהרג עליהן אם גנב ועשק או אנס ופתה בתו של חבירו או שהדליק גדישו וחבל בו וכיוצא בהן ומכלל המצוה הזאת שיושיבו דיינין גם בכל עיר ועיר כישראל ואם לא עשו כן אינן נהרגין שזו מצות עשה בהם … וכי אנשי שכם וכל שבעה עממין לא עובדי עבודה זרה ומגלה עריות ועושים כל תועבות השם היו והכתוב צווח עליהן בכמה מקומות (דברים יב ב) על ההרים הרמים ועל הגבעות ותחת כל עץ רענן וגו' לא תלמד לעשות וכו' (שם יח ט) ובגלוי עריות כי את כל התועבות האל עשו וכו' (ויקרא יח כז) אלא שאין הדבר מסור ליעקב ובניו לעשות בהם הדין

אבל ענין שכם כי בני יעקב בעבור שהיו אנשי שכם רשעים ודמם חשוב להם כמים רצו להנקם מהם בחרב נוקמת והרגו המלך וכל אנשי עירו כי עבדיו הם וסרים אל משמעתו ואין הברית אשר נמולו נחשב בעיניהם למאומה כי היה להחניף לאדוניהם ויעקב אמר להם בכאן כי הביאוהו בסכנה שנאמר עכרתם אותי להבאישני ושם ארר אפם כי עשו חמס לאנשי העיר שאמרו להם במעמדו וישבנו אתכם והיינו לעם אחד והם היו בוחרים בהם ובעטו בדבורם ואולי ישובו אל ה' והרגו אותם חנם כי לא הרעו להם כלל

Ramban on Genesis 34:13:1

(13) And the sons of Yaakov answered Shekhem and his father Chamor with guile: Behold, Chamor and Shekhem spoke to her father and her brothers, but her father did not answer them anything, as his sons spoke in his place about this matter out of respect for him. Since the thing was a disgrace for them, they didn't want that he should open his mouth to speak about it at all. And there is a question here, as [it] appears that they answered with the agreement of the their father and with his counsel, as they were in front of him and he knew that their answer was spoken with guile. And if so, why did he get angry [with Shimon and Levi]? And also, it is not plausible that his will would be to marry his daughter to the Canaanite that defiled her. And behold, all of the brothers answered with guile, [even if] Shimon and Levi did the deed and [yet] the father only cursed their anger. And the answer is that the guile was in telling them to circumcise all their males, since they thought that the people of the city would not do this. And if maybe they would listen to their prince and they would all be circumcised, they would come on the third day when they were hurting and they would take their daughter from the house of Shekhem. And this was the counsel of all of the brothers and it was with the permission of their father. And Shimon and Levi wanted to take revenge upon them and killed all of the men of the city. And it is plausible that Yaakov's anger that cursed them was because they killed the men of the city that had not sinned against him. And what was appropriate for them was that they would kill Shekhem only. And this is what the verse states, "And the sons of Yaakov answered Shekhem and his father Chamor with guile, and they spoke since he defiled Dinah, their sister" – since they all agreed to speak with guile because of the villainy that he did to them.

And many will ask, "And how is it that the righteous sons of Yaakov did this act to spill innocent blood?"
And the teacher (Ramabam) answered in the Book of Judges (Mishneh Torah, Kings and Wars 14:9) and said that the Children of Noach (gentiles) are commanded about laws (dinim), and that is that they set up judges in each and every town to judge about their six commandments. And a gentile who transgresses one of them is killed by the sword. [Likewise] if one sees someone who transgresses one of them and [that a court] did not judge him to kill him, behold, this observer should kill him with the sword. And because of this, all the men of Shekhem were liable for [the] death [penalty]. As behold, Shekhem had [kidnapped] and they all saw and knew [it] and did not judge him (and since they did not enforce civil law, their punishment was death).

And these words are not correct in my eyes. As, if so, Yaakov, our father, was obligated to be prior and merit [to be the instrument] in their deaths. And if he was afraid from them, why did he get angry with his sons and curse their anger several times and punish them and divide and scatter them? Did they not merit and do a commandment and trust in God, and He rescued them?

But according to my opinion, the laws that were counted [as one] of the seven commandments that the Children of Noach were commanded is not only to set up judges in each and every town, but [rather] He commanded them in the laws of theft and fraud, abuse, fair wages, laws of guardianship, rape and seduction, the main categories of damages, injury to ones's fellow, laws of the borrower and lender, laws of purchase and sale and similar to them – like the matter of the laws that Israel was commanded. And he is killed for them if he steals or abuses or rapes or seduces the daughter of his fellow, or if he burnt his hedge and caused injury with it, and similar to this. And in the category of this commandment is also that they set up judges in each and every city – like Israel. And if they do not do so, they are not killed; as this is a positive commandment. … Were the men of Shekhem and all of the seven nations not idolaters, sexually immoral and doing all of the abominations to God? And Scripture yells out about them in several places: "whether on lofty mountains and on hills or under any luxuriant tree" (Deuteronomy 12:2); "you shall not learn to do, etc." (Deuteronomy 18:9); and concerning sexual immorality, "For all those abhorrent things were done, etc." (Leviticus 18:27). But rather [the reason that Yaakov did not want to kill them is that] the matter is not given over to Yaakov and his sons to enforce the law upon them.

But the [actual understanding of the] matter of Shekhem is that since they were evildoers and their blood was considered like water about them, the sons of Yaakov (Shimon and Levi) wanted to take vengeance upon them with the vengeful sword, and they killed the king and all of the people of his city, since they were his servants and submitted to his directive. And the covenant that they should be circumcised was not considered anything in their eyes, as it was to flatter their master. But Yaakov said to them about this that they brought him into danger: "You have brought trouble on me, making me odious," and then he cursed their anger, as they did violence to the men of the city. Since [the brothers] said to them in his presence, "and we will dwell among you and become as one nation," and [the men of Shekhem] chose them, and [yet Shimon and Levi] upended their words – and perhaps they would have repented to God. And [if so] they killed them for nothing, as they did not do any evil to them at all.

But here, having done to Israel something…even though only one of them committed the act—since he is one of the people and since they set upon them first, they are fair game for revenge. Thus with all wars…even though there may have been many who did nothing, this does not set them apart, since they were part of the same nation that did them evil—hence they may be attacked in war. Thus is the case in all wars.