Genesis 34:7
And the sons of Jacob came out of the field when they heard it: and the men were grieved, and they were very wroth, because he had wrought folly in Israel in lying with Jacob’s daughter; which thing ought not to be done. And the sons of Jacob came out of the field when they heard it: and the men were grieved, and they were very wroth, because he had wrought folly in Israel in lying with Jacob’s daughter; which thing ought not to be done.
Shechem's lying with Jacob's daughter Dinah dishonored her, and it was a shameful and grave wrong regarded as folly in Israel that ought never to be done. Jacob's sons came in from the field when they heard of it, and the men were grieved and very angry over what had been done to their sister. Their anger had just cause because of the dishonor done to Dinah.
That grievous anger set the stage for Simeon and Levi's deceptive and violent revenge upon Shechem and his people. That revenge was carried out by deceit and by cruel violence, and although provoked by a real wrong, it was itself a grievous sin. Jacob afterward condemned the fierce and cruel anger that drove his sons to such deeds. In that condemnation he foretold that Simeon and Levi would be divided and scattered in Israel.