What major sin did Moses commit?

Answer 1

None. He committed the minor sin of misinterpreting God's command to speak to the rock (Numbers ch.20). This is spoken of as a more-than-minor sin (Numbers ch.20 and 27, Deuteronomy 32) only in light of Moses' greatness, which made God expect more of him. Other minor infractions, such as delaying the circumcision of his son (Rashi commentary, Exodus ch.4) or expressing anger (Numbers ch.32 and other passages) are not held against him, since in each case he was aware of the problem and had weighed it in light of what seemed best in God's eyes.

In retrospect, Moses was the greatest of prophets (Numbers ch.12, Deuteronomy ch.34).

Another View:

Moses was a man, therefore not perfect. He was to circumcise his son but did not which angered God to the point of looking to kill him:

Exodus 4:24-26New King James Version (NKJV) 24 And it came to pass on the way, at the encampment, 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 Moses' feet, and said, "Surely you are a husband of blood to me!" 26 So He let him go. Then she said, "You are a husband of blood!"-because of the circumcision.

Answer 2

It was because he struck the rock instead of speaking to it (see Numbers ch.20).

It's important to be aware that this was not a "sin" in the sense that we use the word "sin." Rather, it was an error in judgment which caused a diminished sanctification of God's name. See the commentaries (e.g. Rashi and Ohr Hachaim) on the above passage.

Answer 3

he killed one of the guards in Egypt, the guard was beating a slave and Moses pushed the guard off the bridge

Answer 4

Disobey