“when they cut the calf in two and passed between its parts.” A common way of making a blood covenant in the ancient world was to cut an animal in half and then for both parties to walk between the pieces. In the case of the Abrahamic covenant, God had Abraham cut animals in half, but then He put Abraham to sleep and went between the pieces Himself (Gen. 15:9-21).
The killing of the animals and cutting the large ones in half was a form of self-maledictory oath, and was the equivalent of saying, “If I do not keep this covenant, may what happened to these animals happen to me.” Then, if one party did break the covenant, the other party often did seek the death of the covenant breaker. Here in Jeremiah, the leaders who made the covenant then broke it were the subject of divine retribution. God said, “I will even give them into the hand of their enemies…and their dead bodies will be for food to the birds of the heavens and to the animals of the earth.” Those leaders made a blood covenant and then broke it, and God sought their lives. It was acceptable in the culture to have someone else carry out the retribution, as we see here. It was the Babylonians who destroyed Judah.