I take it "the sons of Jacob" refers to the wider group of sons—not just to Simeon and Levi; but to Reuben, and Naphtali, and Asher and the others as well. After Simeon and Levi did what they did, the others came in and spoiled the city—took away the goods.
Of course, he should not have, but Jacob was at least trying to find a political way out of this thing. I am sure that he understood that he was vastly outnumbered by the inhabitants of this land, and he wanted to get out of there with his skin saved. The only thing that I can think of was that God intervened and allowed Jacob (and all of his family) to escape with his hide. Otherwise, something very terrible might have occurred to the very group that God was beginning to form, that would be the fathers of their nations.
