Shechem son of Hamor the Hivite, saw Dinah, he took her and raped her.

Genesis 33:18-34-17
For a hundred pieces of silver, he bought from the sons of Hamor, the father of Shechem, the plot of ground where he pitched his tent. There he set up an altar and called it El Elohe Israel. l Elohe Israel can mean El is the God of Israel or mighty is the God of Israel.
Now Dinah, the daughter Leah had borne to Jacob, went out to visit the women of the land. When Shechem son of Hamor the Hivite, the ruler of that area, saw her, he took her and raped her. His heart was drawn to Dinah daughter of Jacob; he loved the young woman and spoke tenderly to her. And Shechem said to his father Hamor, “Get me this girl as my wife.”
Because their sister Dinah had been defiled, Jacob’s sons replied deceitfully as they spoke to Shechem and his father Hamor. They said to them, “We can’t do such a thing; we can’t give our sister to a man who is not circumcised. That would be a disgrace to us. We will enter into an agreement with you on one condition only: that you become like us by circumcising all your males.
Dinah is raped by Shechem, leading to the worse crime of murder by Dinah's brothers. The story shows Dinah as silent and powerless, where a modern. The western account might well focus on the victim and her feelings. But the issue that concerns the writer/editor here is one of tribal and national survival.  The terrible vengeance exacted by Dianah's brothers for the insult to their sister shows the need for the law to limit revenge.
Why?
People of Shechem did the wrong thing, they could not rape a woman.
This is a revenge action from Dinah's brother, they were angry and fury about their sister Dinah' suffering. It is a shame for their family.
Jacob's son deceived Hamor the Hivite and make an agreement to try to kill them.
How?
Evil things always make another evil circumstance. Don't have any evil mind in our hearts.