Why is the killing of the man by Prophet Musa (peace be upon him) considered a sin, even though he killed him by mistake and did not intend it?

Moses (peace be upon him) did not intend to kill the man, as evidenced by his use of a 'nudge' (wakza) which does not usually kill, but rather intended to repel injustice. Moses considered this a sin and sought forgiveness for it, even though it was an accident and not intentional. This was due to his greatness and compassion, and because prophets greatly magnify any action that resembles disobedience or goes against what is better.