Well, it is not the problem with Bitcoin but the human involved. There are also cases whereby people buy and sell drugs or involve in any illegal activities using fiat.
ya absolutely agreed , its not the problem of currency , its issue related to people, all the terrorism ,drugs mafia support , trafficking all happens in fiat, they don't use digital currency that much so they can't get thought and if they use it belong to rich person who cannot be doubted because of him/her already having that much amount of money.
illegal deeds depend upon human morals and a non living thing whether bitcoin or fiat is not be blamed for it.