Jamieson, Fausset and Brown on Exodus 2:12

12. he slew the Egyptian, and hid him in the sand--This act of Moses may seem and indeed by some has been condemned as rash and unjustifiable--in plain terms, a deed of assassination. But we must not judge of his action in such a country and age by the standard of law and the notions of right which prevail in our Christian land; and, besides, not only is it not spoken of as a crime in Scripture or as distressing the perpetrator with remorse, but according to existing customs among nomadic tribes, he was bound to avenge the blood of a brother. The person he slew, however, being a government officer, he had rendered himself amenable to the laws of Egypt, and therefore he endeavored to screen himself from the consequences by concealment of the corpse.