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Why did the angel of God want to kill Moses?

What had Moses done to warrant such punishment?

Thanks for your question! 

Your are referring to the incident recorded in Exodus 4:24-26. I think the answer hinges on the fact that Moses, as the chosen leader of God's people, is held to a very high standard. In James 3:1 (NIV) we read,   "Not many of you should presume to be teachers, my brothers, because you know that we who teach will be judged more strictly." Moses was given vastly  more responsibility than that of a teacher. See the severity of this responsibility in Numbers 20:10-12.
The incident recorded in Exodus 4:24 is in a similar vein. Moses' younger son was born perhaps on the way to the promised land and Zipporah, it seems, did not want to circumcise this child, Eliezer. Moses, gave in to his wife's refusal and God sent his angel to  persuade Moses of the seriousness of the situation, perhaps as he did with Balaam in Numbers 22:22-33. 
There was a law in existence before the "Law of Moses." We see hints of it from Genesis 3 and 4 to the sacrifices made by Noah and Abraham. In the Law of God before the time of Moses, circumcision was a sign of the covenant between God and His people. It had been so since the time of Abraham. To neglect this law meant exclusion from the covenant and people of God. What a lesson! Teachers of God's law must themselves be obedient to that law. How could Moses stand up for righteousness with this sin of omission on his conscience?
 After this incident, Moses would know with absolute clarity that God means what He says. We know what happened to David after his sin with Bathsheba.  Even though he was forgiven, the moral consequences to his leadership in Israel were horrendous. God saved Moses from such a mistake, impressing on him the life and death importance of obedience to His Law. See 1 Samuel 15:22,23!
 One wonders, in the light of such incidents as this, how some followers of Christ can so easily set aside the commandment to be baptized. See Matthew 3:13-15 and Mark 16:16. Like circumcision, baptism is a sign of the covenant between God and His people. It is a commandment upon which our salvation depends!
 See: Why Baptism Really Matters 
 I hope this has been helpful.
 God bless!
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