‘And when they heard it the sons of Jacob came in from the countryside. And the men were very grieved, and they were furiously angry because he had wrought folly in Israel in lying with Jacob's daughter, which thing should not be done.'
When the news reached the sons of Jacob their anger reached fever pitch. In their eyes what had been done was unforgivable. It was a grievous sin. They came in from the countryside bent on doing something about the situation.
“They were furiously angry because he had wrought folly in Israel.” The phrase ‘wrought folly in Israel' refers to what is seen as the most grievous of sins. It usually has in mind sexual sin of the worst kind but is also used of Achan's sin in retaining what was devoted to Yahweh (see Deuteronomy 22:21; Judges 20:6; Joshua 7:15). The word for folly is nebala, seemingly an expression for what is basically sacrilege.
It has been suggested that the reference to ‘Israel' might suggest that the last part of the sentence was a note appended later to emphasise the depth of the sin in order to explain why the brothers behaved as they did, and that may be so. On the other hand the brothers had fresh in their minds the dedication of the altar to ‘God, the God of Israel', which could explain the use here with the tribe having a new sense of their identity as ‘Israel'. In other words they saw the sacrilege committed on Dinah in the light of the recent dedication of the tribe as Israel and it made the sin even more heinous. They had become established under a new name in the eyes of their God and now almost immediately this slight on the new name had occurred. Shechem had taken that which was devoted to Yahweh. Thus the phrase ‘folly in Israel' may well have arisen from this incident.
(That the use of the name Israel is now fairly regular comes out in Genesis 35:21-22; Genesis 37:3. Thus its use here when the setting up of a permanent altar to ‘God, the God of Israel' has recently taken place is to be expected, especially in a context referring to sacrilege).
“Which thing should not be done.” This re-emphasises the awfulness of the crime. It was clearly felt very bitterly.