There is a curious debate as to whether the sons of Yaakov were involved in a plan of deception or not. Shchem and Hamor come to Yaakov and his sons and request that they allow the families to intermarry. The children respond that they cannot marry with uncircumcised people. They make a deal that the people of Shchem will circumcise themselves and then they will allow marriage (or consideration of it). Then, on the 3rd day after the circumcisions when they were in the most pain, Shimon and Levi killed everyone out.
It looks like the whole deal was a plot to weaken them and set them up as easy prey in a way that they would not be able to fight back effectively.
I would like to suggest that there was no plot here and no deception. The brothers were really making a deal. If it was a plot, why were Shimon and Levi the only ones who went and killed everyone? Where were the others?
There was no plot. They really were making a deal. I think that as the deal was panning out and the people of Shchem were fulfilling their end of it, Shimon and Levi woke up and asked themselves how they could let this happen. They realized that the whole deal was wrong and corrupt and only harms their interests. So they got up and reacted.
This is similar to what we see happening by the story of Chushim Ben Dan, when Yaakov dies. We know the stroy that they were bringing Yaakov for burial and Esau stopped them claiming the plot in Hebron was rightfully his, not Yaakov's. Naftali ran back to Egypt to retrieve the document of sale as proof. In the meantime, Chushim, who was deaf and not clear as to what was happening and holding up the funeral, killed Esau and the funeral proceeded. The brothers entered negotiations and Chushim realized that just the delay itself, the negotiating itself was a desecration fo Yaakov. Sure, we can prove our point by bringing the documents, but just responding to Esau was a desecration of Yaakov's honor, so he stood up and took action.
Shimon and Levi realised that the negotiations between the brothers and Shchem were harmful to their interests and a continued desecration of Dina's honor and their honor. They stood up and took action. And that is why it was only them. There was no plot and deception taking place.