Question: "Why was God going to kill Moses in Exodus 4:24-26?"
Answer: God was going to kill Moses because of sin. The sin of Moses in Exodus 4:24-26 is not stated explicitly, but the surrounding events give substantial clues as to the nature of Moses’ transgression. God had instructed his messenger to warn pharaoh to free Israel or pharaoh would lose his firstborn son (Exodus 4:21-24). Moses had been specially groomed by God for eighty years for this mission, and now the time for action had come.
Moses was to lead his people out of Egypt and to be an example to pharaoh’s house, to the nation of Egypt, and to all the nations that heard of those happenings (Exodus 18:10-11; Joshua 2:10-11). Accordingly, Moses’ personal life had to be in order before he could direct the spiritual lives of the Hebrew people. It seems that Moses had neglected to administer the sacred rite of circumcision, the act that symbolized the Almighty’s covenant with His chosen people.
Perhaps this was the result of pressure from his surrogate Midianite tribe. It is also possible that he was persuaded by Zipporah not to circumcise his son, since she apparently found the practice revolting (4:25). This would explain her violent outburst; she felt that she had saved her husband from death by shedding the blood of her son. Whatever the cause, Moses’ outstanding sin made him unfit to serve as a spiritual leader, and the situation had to be rectified before he could carry out his mission effectively. Indeed, as soon as Zipporah performed the act, the Lord “let him go.” In summary, God was going to kill Moses because Moses was supposed to teach the Israelites God's Law, yet Moses was not obeying God's Law himself.