Moses was the reincarnation of Abel. In the Torah, Moses is called "man." The Jews called him that when they said, "This man Moses, we do not know what happened to him." So when the Torah says "one who hits a man," it means Abel and his reincarnation, Moses.
The first murder in the Torah is Cain killing Abel. Since Abel was later reincarnated as Moses, and Cain was reincarnated as that Egyptian whom Moses killed, then this was the first time. The second time happened when Cain was reincarnated as Korach. Moses killed him again by making the ground swallow him.
One can notice how precisely the judgment is meted out. Cain hid Abel in the ground. Corresponding to that. Moses covered the Egyptionan with sand. With Korach, Moses made the earth swallow him to make the message very clear.
Art: Cain and Abel by Titian