Moses had apparently put off circumcising his son, which was a violation of God’s command given in Genesis 17:10 – 13: “This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised. And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you. And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant.”

In essence, God was going to kill Moses because Moses was supposed to teach the Israelites God’s Law, but he himself was not obeying God’s Law