The story of Dinah’s abduction and rape does not stand alone. Dinah was merely the third in a line of Jewish women who were presented as “sisters” – and then taken by non-Jewish men. The difference is that Dinah was actually the first of these women who was actually not married and thus, in the local customs, available.
Yitzchak and Avraham had deferred to the law of the land, which is why they lied about the true nature of their relationship. And it never ended well: in all cases the lie was exposed – and the justification for the lie was also debunked, as Avraham and Yitzchak were not killed so Sarah and Rivkah could be freely taken by the local lord. The Torah presents the story without commentary, but the events described show that the lies were both unnecessary and damaging. The whole reason to lie was to avoid being killed – but when the lie was exposed, no harm befell them!
When Dinah was taken, had the family reacted just as Avraham and Yitzchak had done, then she would have married Shechem, and that would have been the end of that. After all, local customs cannot be ignored, and a person needs to be realistic about the power imbalances: a single family cannot survive by earning the enmity of an entire region. Or so our patriarchs, including Yaakov, thought.
But Shimon and Levi had different ideas, and they were crucial for the forming of the Jewish nation. Shimon and Levi made a decision: there is right, and there is wrong. And principle sometimes trumps realpolitik. They were the first Jews to say that Jewish law and custom is more important than someone else’s law and custom.
See it from the perspective of Yaakov’s sons. Their grandmother had given her consent. Their father not only had Rachel and Leah’s consent, but he worked for 14 years to earn his wives. It is simply not acceptable for a man to seize a Jewish woman off the street. So they reacted with a sword, dividing the Jewish nation from the rest of the world. Their actions were ferocious, and they clearly let their anger get the better of them – but they got results.
I would go so far as to suggest that Shimon and Levi did what Avraham and Yiztchak failed to do: stand up for what they believe was right, by forcing other people to accommodate to the Jews, and not the other way around. This was an essential step for the Jewish people to grow into an independent nation: the confidence that our own laws and society are good and proper and true, even for other people. It was a corrective act on several levels; not only is this the last time the Torah tells us of a non-Jew taking a Jewish woman, but it is also the end of Jewish men lying about the identity of their wives. Once Jewish men learned to stand up for the honour of Jewish women, it became possible to start to build a nation.