What was the reason for commanding them to slaughter a cow?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. Ibn Sīrīn narrated from ʿUbaydah who said: There was a man among the Children of Israel who was infertile and had no children, and he had a lot of wealth. His nephew was his heir, so the nephew killed him and carried him at night to another area, placing him at the door of another man. In the morning, the nephew claimed that the other man had killed him, causing the people to take up their weapons and ride out to meet each other (for conflict). They came to Moses and told him what happened, so he commanded them to slaughter the cow.
2. Al-Suddī narrated from his teachers that a man from the Children of Israel had a daughter and a poor nephew. The nephew asked for the daughter's hand in marriage, but the man refused. The nephew became angry and said: "By Allah, I will kill my uncle, take his wealth, marry his daughter, and take his blood money." He went to his uncle and said: "Merchants have come to one of the tribes of the Children of Israel. Go with me to take some of their merchandise, perhaps I can make a profit." He went out with him, and when they reached that tribe, the nephew killed him. The nephew then returned, pretending to search for his uncle, acting as if he did not know where he was. The tribe gathered around him, and he accused them of killing his uncle, crying and lamenting, "O my uncle!"

Abū al-ʿĀliyah said: The one who asked Moses to ask Allah for clarification was the killer himself. Others said: Rather, the people gathered and asked Moses, and when he commanded them to slaughter a cow, they said: "Do you take us in mockery?"