Bitcoin has been used for illegal activities ever since the first iteration of silk road came out. Its anonymity and irreversibility guarantees it will always be lucrative for illegal trade and criminals to use.
Some see this as an advantage rather than disadvantage but I guess it depends on what situation you are in, for example if you get robbed or hacked its a huge disadvantage.
Bitcoins is a digital currency everyone has access to it
Good and evil are the two side of Same coin , where their is good it has to be nullifyed by evil so to make the thing in equilibrium ,
It's same in the case of bitcoins:-
Many people use it for trading for business purposes for investment and many more things it has many great advantages as a digital currency and people use it to its full potential
While there are many people who use bitcoins to buy drugs and also supply weapons as no one can see their identity all we can see is an wallet address
They are using one of the advantages of Bitcoin transaction
For their are no rules for using bitcoins we can't say it is ileegal on digital scale ,
But yes when we see the consequences and the things they are buying it is illegal and this is one of the reason that many countries are not accepting bitcoins