Exodus 24:21 And the LORD said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go. 22 And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn: 23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn. 24 And it came to pass by the way in the inn, that the LORD met him, and sought to kill him. 25 Then Zipporah took a sharp stone, and cut off the foreskin of her son, and cast it at his feet, and said, Surely a bloody husband art thou to me. 26 So he let him go: then she said, A bloody husband thou art, because of the circumcision.
The problem was that Moses had failed to circumcise one of his sons.
This meant that the son was cut off from the covenant that God had made with Abraham, and consequently could not begin to serve God.
Genesis 17:14 And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
This was in direct contradiction to the mission for which God was sending Moses into Egypt. – ‘Let my son go, that he may serve me’.
As it appears that only one son needed circumcision, we might reasonably suppose that it was the youngest.
From Zipporah’s words, we might also reasonably suppose that she had objected to his circumcision and Moses (being meek) had deferred to her.
What is not clear was whether the angel was attempting to kill Moses or his firstborn or the uncircumcised child.
The penalty for Pharaoh’s refusal to let Israel go was the death of his firstborn.
This gives a reason for thinking that the firstborn was the angel’s target.
Death would seem to be being inflicted by some visible illness.
Zipporah clearly recognised that the solution was to circumcise the boy.
Moses’ inactivity might point to his being the imminent death, but if his firstborn was dying, he may have been fully occupied tending to him.
We can be reasonably sure that it was the youngest who needed to be circumcised, and that Zipporah had been instrumental in preventing it.
We can’t be sure whether the angel was going to kill Moses or his firstborn although the latter would seem to fit with God’s message to Pharaoh.
Throwing the foreskin at Moses’ feet, was both an act of abhorrence and submission on the part of Zipporah.
‘A bloody husband thou art’ arose from the shedding of her son’s blood in circumcision because of her husband being part of the Abrahamic covenant.
I hope you find this helpful.