Ramban answers that the Chivites were very violent people. Life was worthless to them. The only way to deal with such people is with a sharp, vengeful knife. The citizenry who were murdered were King Shechem’s slaves and would listen to anything he said. The covenant which they made with Yaacov and his family was viewed as worthless and temporary. They entered the covenant only as a temporary measure until they could confiscate all of Benei Yisrael’s property. (see also Nechama Leibowitz)
As part of this discussion Ramban – Nachmonidies quotes Rambam – Maimonides. Rambam (Melachim 14-9) says that all of the people had incurred upon themselves the death penalty. The seven Noachide Mitzvot include a commandment to set up a justice system. Transgressing any of these Mitzvot incurs the death penalty. A gentile who steals can be put to death in a Bet Din. The people who were subservient to Shechem and Chamor did not try them for the rape of an innocent girl. Therefore, it is a Mitzvah to murder all of these people. I have heard this stated by Rabbi Yaakov Yosef Z”L with regards to our present enemies. (Rabbi Yakov Yosef was questioned by the Israeli police on this matter)
Ramban – Nachmonides feels that Rambam – Maimonides is very mistaken. If Rambam is correct Yaacov should have gone with his sons to kill all of the people. Instead he was angry with Shimon and Levy and cursed them.
Ramban’s approach is as follows:
It is my opinion that the Noachides are not only commanded to set up a justice system but must enact civil laws. These laws include theft, price fixing, racketeering, timely payment of employees, laws regarding borrowed property, rape, seduction, damages, borrowing and banking laws as well as laws of trade. All of this is similar to Jewish civil laws. A gentile who transgresses any of these laws would incur the death penalty…. Part of these Mitzvot is to set up a justice system. However, if they did not set up a justice system they do not incur the death penalty. Only transgression of a negative commandment would incur the death penalty.