JOEL - Here's the relevant scripture:

"And it came to pass in those days, when Moses was grown, that he went out unto his brethren, and looked on their burdens: and he spied an Egyptian smiting an Hebrew, one of his brethren.
And he looked this way and that way, and when he saw that there was no man, he slew the Egyptian, and hid him in the sand." (Ex 2:11-12)

It could be argued that Moses was defending a fellow Hebrew who was being beaten, so most people would not consider it murder. It is up to God to judge something like this; apparently God did not judge him as a murderer, otherwise he would not have been called a prophet. Also this happened before he became a prophet.
It is significant that Bible commentators also point out that the Bible nowhere condemns Moses' action nor calls him a murderer. In the eyes of God, and according to the law of most, if not all, human societies, this killing was excusable homicide, not murder.