[Eomer of the Rohirrim]

That quotation doesn't mean that the misery is inherent. They are placed in this dark, desolate life; and the misery springs forth very quickly from this arrangement. That line is there to show that the Orcs were slaves and not willing workers. The hatred is deep inside and has been since the start of their lives; but it doesn't suggest that the Orcs themselves are based on the hatred.