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.
First, according to the study, bitcoin is used only as little as ten percent for any illegal purposes precisely because it is not completely anonymous. To commit crimes, a coin like monero is used, which by the level of anonymity far exceeds bitcoin. Using the public address of the desired bitcoin purse and the contents of this purse, that is, what coins are there and how much, and also having the opportunity to trace all the transactions of this purse, law enforcement agencies can go to the exchange or exchanger, where the owner of this purse, performing operations with the crypto currency, identified itself. Thus, it will be installed. At the request of law enforcement agencies, exchanges and exchangers will be required to provide an opportunity to take pictures of these financial transactions, as well as provide other data that will serve as evidence for the court.