Bitcoin will be used for good as well bad things the same way it happens for several years with fiat, soo nothing new here, besides its harder to trace the people who are under the dark marketplaces.
I think although Fiat is also used for illegal activities as well but I think it will not be as perfect as bitcoin, because you can be easily tracked when you use fiat to transact. with bitcoin you can not be tracked and there is no evidence of when you really use for illegal activities