When a Party Vastly Exceeds their WBL

[tainsouvra]

I think it's completely reasonable for the DM to take control of a raging barbarian.

While you are welcome to houserule that in, there is nothing in the rules about the player not being allowed to play his character and one rule that would unequivocally prevent what happened from being legal. Remember, the rules specifically permit the barbarian to end his rage at will, which means that the DM making the barbarian not able to take control of his actions again after felling his opponent is completely contrary to the ability's description. At a bare minimum, the player is completely justified in saying "my character stops raging". Additionally, it is generally bad form for the DM to take control of a player's character when it isn't strictly necessary. In effect, it's using Rule 0 to stop a player from playing the game.