First thing which comes to my mind about using cryptocurrencies in crimes are money laundering and transactions on darkweb. It was said that from 2019, 829 million dollars in Bitcoin has been spent there. Security system which leaves much to be desired and lack of any regulations are the main reason, nothing will change without these key factors. There are associations set up to prevent such activities for instance Coinfirm, which recently expanded their offer by reclaiming stolen cryptocurrencies. Hope there will be more and more companies in the future like that one, which use blockchain technology to solve serious problems. If you want to know more, you can visit their website: https://www.coinfirm.com