"And the sons of Ya'akov spoke to Shechem ... cleverly (Rashi and all the commentaries)". What was so clever about their condition?
Seforno: They were convinced that either Shechem and Chamor, or the townspeople 1 , would never agree to circumcise, in which case they would take their sister and leave.
Ramban: They assumed that the townspeople would not consent to circumcise, and even if they would, they would easily be able to take Dinah and leave. 2
Hadar Zekenim: They said "we would be willing to be like you - just one cannot return Orlah that was cut, so you must circumcise to be like us."
Bechor Shor: It was unreasonable that the townspeople would circumcise to enable Shechem to marry her. However, Shechem and Chamor convinced them that they will gain monetarily through this (refer to 34:25:6:1).
Which would be perfectly permissible, since they had defiled her, as the Pasuk specifically states.
1. Seeing as their clever answer was given with Ya'akov's consent, why was he angry at them when they carried it out? 2. How can Ya'akov have agreed to Dinah marrying a Nochri? 3. How do we explain the fact that all the brothers agreed on the plan of action, yet it was only Shimon and Levi who carried it out and it was them alone whose anger Ya'akov cursed?
Ramban #1: Ya'akov gave his consent to the initial plan, 1 but not to Shimon and Levi, who took it upon themselves to avenge their sister's shame and to kill all the inhabitants of Shechem (an act that the brothers had not envisaged.
Ramban #2: Even if they had killed Shechem, Ya'akov would not have ben angry, since their plan had been formulated due the fact that 'they had defiled their sister' (as the Pasuk specifically states); and it was the fact that they killed all the inhabitants of the town who had done them no harm, that aroused his ire.
How could the sons of Ya'akov who were Tzadikim, kill innocent people?
Ramban (citing Rambam): Because a Nochri who does not keep one of the seven Mitzvos Bnei No'ach (of which judging is one), is Chayav Misah. Consequently, since the whole of Shechem knew that Shechem stole Dinah, and failed to judge him, they were all Chayav Misah. 1
Ramban: To take revenge against the people of Shechem, who were all wicked 2 and whom they killed together with their leaders who perpetrated the crime. As for the B'ris Milah, they only circumcised in order to follow the lead of Shechem and Chamor, rendering it meaningless.
See Ramban, who disagrees with this for a number of reasons, mainly because the Mitzvah of Dinim constitutes initiating a constitution with civil laws, incorporating laws regarding theft, overcharging, and business ethics, etc., and not judging per se. Moreover, he argues, all the nations of the world were indulged in idolatry and adultery (see for example, Devarim 12:2 and Vayikra 18:22, respectively), yet nowhere do we find that the Avos took them to task.
And Chayav Misah for other sins that they had perpetrated (See footnotes of R. Chevel), though that was not their main reason for killing them.
רש"י: הכתוב אומר שלא היתה רמיה: אולי אלה דברי האחים עצמם?
גור אריה: פשוט שהם לא אמרו "אשר טימא" כי אז היו חמור ושכם מבינים את כוונתם, אלא זה דברי הכתוב
רש"י: הכתוב אומר שלא היתה רמיה שהרי טמא את דינה אחותם: מנין שזו כוונת הכתוב?
גור אריה: כיון שאלה לא דברי האחים אלא דברי הכתוב, בהכרח שזו כוונת הפסוק.
כיצד הותר להרוג את כל העיר בגלל חטאו של שכם?
גור אריה: בני ישראל והכנענים נחשבו כשתי אומות, ומכיוון שהם הרעו לישראל תחילה, מותר לישראל להלחם בהם. 1
מבואר בגו"א שכאשר עמים נלחמים זה בזה, מותר לפגוע בכל העם האויב ולא רק במי שהרע לנו.
כיצד נלחמו ישראל בשכם והרי התורה ציוותה לפתוח בשלום?
גור אריה: צריך לפתוח בשלום רק כאשר העם השני לא עשו לנו רעה [וכאן הם חטפו את דינה].