I'm guessing this one got pulled tried to disengage and stood there spamming arcane shot wondering why the DK hadn't dropped dead yet. The DK was smart enough to pop antimagic shield and absorb half the damage of Arcane shot and got him with Obliterate.
And btw Aspect of the Iorn hawk gives us 15% damage reduction and 25% increased attack power not 10% increased attack power @ryudaraku.
DK's are fine. They have a similar kit to warriors. they are supposed to be sturdy. if you see one in Blood presence the best way to fight him is to keep him off you. Disengage if you get pulled. If disengage is on CD then scatter shot strafe circle away from his partner, concussion shot. and dps him while keeping concussion shot applied and he won't even touch you, unless you let him.