Even in death he looks angry.
After he dies, his corpse remains as a lootable 'coffer' to obtain raid rewards. If you receive a unique drop (Achto or an Ability Codex) you will not receive a main drop from Beastmaster Durzag.
After you have defeated Beastmaster Durzag, you are now able to fight Yakamaru
The chance of receiving better drops is increased by the Luck of the dwarves.