‘And it happened on the third day, when they were sore, that two of the sons of Jacob, Simeon and Levi, Dinah's brothers, took each man his sword and came on the city unawares and killed all the males. And they killed Hamor and Shechem his son with the edge of the sword, and took Dinah out of Shechem's house and went out.'
The third day was when the circumcision operation laid men lowest and they were feverish with their wound. Then it was that Dinah's brothers Simeon and Levi moved in to exact justice and demand blood to expiate the sacrilege against Dinah. They would be accompanied by their retainers, possibly supplemented by other tribal members, but because of their status as full blood-brothers to Dinah it was seen as their right and responsibility to exact punishment.
The other sons of Leah are not mentioned. Simeon and Levi were seemingly the most warlike of them and most suitable for the enterprise, and they would appear to have been appointed by general agreement to carry out the enterprise. (Jacob will later decry the attributes that made them seem so suitable (Genesis 49:5-7).)
No one in the city, which would be an open unwalled city, was prepared for the assault and inevitably the men were caught unready in no condition to put up a good fight. It is specifically stressed that Hamor and Shechem were put to death. This was necessary expiation. At the same time Dinah was released from her ‘imprisonment'.
“They killed all the males.” It was a bloody business, but this was necessary to prevent retaliation.