Why did God choose the tribe of Levi to be the priests?

This is particularly difficult because we cannot know why God does what He does (Isaiah 55:9).  The man Levi himself was not deserving of this honor, since he along with his brother murdered a whole city (Genesis 34:25).  The only explanation is that since Moses was from the tribe of Levi and he was chosen by God to deliver the Israelites and give them the Law (Exodus 3:6-10).  This is the only thing that makes sense when looking at the scriptures, since God doesn’t really tell us why He chose this tribe.