Since Bitcoin is a completely decentralized currency, there is no way to know when and how much money is being sent to whom, both the sender and receiver remain anonymous. So Bitcoin is used to buy and sell various criminal and illegal products. So various arms smuggling activities are paid through Bitcoin. Also, all transactions on the dark web are done through Bitcoin. Which is very sad for us
You are mixing things up my friend, for the fact that Bitcoin is decentralised doesn't mean its transactions are untraceable. Decentralisation of Bictoin only means that no central authority can control it, not even the creator, which is a very good one. But all transactions are traceable in a normal mechanism. The issue starts with blenders, mixers, tumblers and others, they are the bad actors here. They will make sure that the Bitcoin you received or sent are not traceable anymore, so they can use it for all sorts of illegal activities afterwards, and that will even be to and fro of the transaction not minding the amount they will pay those who are making the money anonymous for them since it is for illegal purpose. Although some people mix their Bitcoin without any bad intentions, however, their percentage may not be so much.
You can see that Bitcoin itself has never been bad but the desperately wicked minds of people are attracting the bad name to it for their selfish gains. I hope this can stop one day, it will be nice if there are crypto police that are highly equipped to trace such transactions and pull them down. This will also discourage people from patronising them. But before you know it, you will hear people defending such services claiming that some people use those services for their privacy to as well