Ethically, there's a difference between profiting incidentally from criminal activity because a fraction of your customers happen to be criminals, and having a business model which is viable largely due to criminal activity.
That difference being directly attributable to the criminal activity. If your business model is killing people and taking their wallets, you are unethical because you murder and steal, not because you have more money in your pocket after you do it.
There was an idea floated a while back about why 51% of the miners couldn't just disavow the Bitcoin that FBI seized from Silk Road users. The answer was that it would completely unravel the currency, and impose an ethical obligation on everyone to examine the source of coins before accepting a transaction.
Gold has no smell. If it did, it would all stink. Attaching ethical considerations to money itself places an additional transaction burden in commerce, such that even a tiny cost can grind everything to a halt, in much the way that a pinch of sand in your oil pan can destroy your engine.
Think about the ethical consequences of currency that is not completely ethically neutral, and compare with the consequences of one that is. In the one case, money laundering happens, and you cannot reasonably prevent it. In the other, black markets flourish as white markets fail, because currency can flow only between them in only one direction.
I'd argue that running a business providing services which are attractive primarily to criminals, like anonymously converting one means of payment to another for a 50% margin, is unethical even though money transmission per se isn't. YMMV
