I have seen some news regarding hackers, terrorists, and other criminals using bitcoin for illegal trades.
How the police can find about these transactions?
Even if is hard to track illegal transactions, I don't think that Bitcoin can be a danger to society, since the problem is the society and its people.
Since the beginning of the internet this is happening ( the bad people committing crimes) and no one is willing to shutdown internet because of that, so in that matters Bitcoin should be equivalent. I think those illegal transaction are one of the reasons some governments and the media are trying to depreciate bitcoin, but the problem is not Crypto/blockchain or Bitcoin, is just the society.
Bitcoin is decentralized and no can even control or manipulate it, through it, anyone can have it. Thus making it possible to become a medium of illegal transactions, as what I know, bitcoin is currently beem used in the darkweb, we ain't know what is happening out there but bitcoin is used in it. The main problem is the people itself, there are good and bad people existing in this planet. If bitcoin will be banned because of this, well then many will protest because even fiats are used in illegal ways. Bitcoin s purpose why it was created is for the people, so we hope that we should use it wisely.