Dark marketplaces are mostly considered bad but I think that they can be good if you think about it, its not that they are doing a bad job in the bitcoin economy as a example and it made the bitcoin also more popular than ever right ?
The people who say that darknet market places are bad are the people who are affected in a not so good way by it. The government would say its bad, maybe the banks would say its bad because they couldnt participate in it, or any business that loses some revenue because of the darknet will say its bad.