Why did God kill Uzzah ? - 2 Samuel 6:6-7 6 When they came to the threshing floor of Nakon, Uzzah reached out
  and took hold of the ark of God, because the oxen stumbled. 7 The
  Lord’s anger burned against Uzzah because of his irreverent act;
  therefore God struck him down, and he died there beside the ark of God.


After reading those verses, I often think this was over-reacted. Maybe Uzzah wasn't thinking badly, maybe was he just taking care of the ark of God ?

So what are the deep/fundamental reasons God killed him ? 

What's your opinion ?
The implication of Numbers 4:15 is that God forbids non-priests from touching holy things like the Ark of the Covenant.  Apparently, God killed Uzzah because he, not being a priest, touched a holy thing contrary to God's commandment.