For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I feel that Moses was wrong. The Bible says 'Thou shall not kill'.
'Thou shalt not murder', not 'Thou shalt not kill'.
Moses' actions could have been pre-meditated murder, as it does not appear that he immediately stepped in to stop the beating or save the Hebrew, but waited until no one was around. They could also be construed as the actions of a judge. The Egyptian smote the Hebrew, Moses smote the Egyptian. The word is the same. We do not know the severity of the Egyptian's beating, - he could even have died. In the law given later, vengeance is to be taken on any master who beats his slave fatally (Ex 21:16-27). Also, the slavery in Egypt was of the form of kidnapping and slavery(I Tim 1:10), a crime deserving of death. These people did not willingly enter a contract of servitude, nor were they captured in war. Egypt invited the Israelites in, then later enslaved them.
There simply is not enough detail in the text to determine the exact scenario - but Moses acting as judge would fit better with the response of the Hebrews (Ex 2:14). They did not run away screaming 'murderer', but rather admonished him, 'Who made you judge over us?'. God later does indeed set Moses up as judge over them (Ex 18:13-27).
As such, a case of Moses acting on his own authority as judge fits far better than Moses murdering someone.