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
You are wrong, fiat ilegal activities are covered with fake companies, with employers, soo they do clean the money they earn from those and you cant trace, otherwise we wouldnt have criminals under those activities, sure bitcoin does make it more easy maybe, but those people will need to exchange the bitcoins into fiat, and doing those they get exposed easy.
It has possibility that bitcoin will used in illegal since we do not give our personal information even you are using fiat in online using your bitcoin wallet you can not easily trace if anyone are looking for your transaction.