Sforno on Genesis 34:7/ספורנו על בראשית ל״ד:ז׳

1ויתיצבו האנשים ויחר, the sadness and sorrow described by the Torah as the brothers’ reaction pertained to the despicable deed that had been perpetrated against a member of Yisrael’s family. This was in spite of the fact that whereas Jews do not rape unmarried virgins, gentiles are known to be guilty of this. Seeing that even in gentile circles it was taboo to rape the daughters of prominent citizens, the Torah adds the words וכן לא יעשה, that such a thing was totally unacceptable. Their anger, ויחר, therefore was turned against the perpetrators and against those who had condoned the act.