When I was 16 years old, I asked the same question of our Presiding Overseer™. He told me what a great question it was, and it would be good to practice my research skills and find out the answer for myself. (Which means, when translated, "Damned if I know. I'm too busy to answer all these questions about the Bible, I've got Judicial Committees™ to run and people to DF. Look it up yourself and let me know.")
Exodus 4
24 Now it came about at the lodging place on the way that the LORD met him [Moses] and ( AB ) sought to put him to death.
25 Then Zipporah took ( AC ) a flint and cut off her son's foreskin and threw it at Moses' feet, and she said, "You are indeed a bridegroom of blood to me."
26 So He let him alone. At that time she said, "You are a bridegroom of blood"--because of the circumcision.
There was a QFR on that scripture that basically said that the "him" in the scripture was not clear as to whom Jehovah intended to kill (I guess he's not picky). The QFR suggested that "him" may have referred to Moses' son, because circumcising the son resulted in Jehovah's changing his mind about killing anyone (ie, he got another foreskin trophy and was happy again). Looking at the context, this is what vs 23 says: 23 "So I said to you, ' ( Z ) Let My son go that he may serve Me'; but you have refused to let him go Behold, ( AA ) I will kill your son, your firstborn."'"
The supposed reason for Jehovah's seeking to put Moses/his son/ (somebody?/anybody?) to death was because of Moses' failure to keep the Abrahamic Covenant of circumcision. It meant that the child was not properly dedicated to Jehovah, so somebody had to die (or get the end of their penis whacked off with a flint) to set things straight.