Bitcoins like fiat currency can be traced back to the sender if he had used a website (LocalBitcoins, PayPal, BitStamp, MtGox etc.) which might link to your real identity. So leaving no traces behind is mandatory, more so if you have received them in payment in exchange for carrying out an illegal activity. This handy website will take care of your worries and wash your coins. The service is available at the given onion address here: (fogcore5n3ov3tui.onion).