I think that we see that Moses is identifying with his Hebrew origins. This has an impact on him, as he sees the injustices being done all around to his people. He probably had lived with seeing this for as long as he can remember, but at this point he has an emotional reaction to the events around him (motivation) and kills the taskmaster. This shows that Moses, has a character that allows him to sin, just like the rest of us. Unfortunately it also shows that he was impulsive and did not think things through very clearly, for he is forced to leave Egypt a wanted man. At this point Moses is not ready to be the leader he is to become.