The hitman was a conman for a murder on a fictitious person. While he fully believed he was committing a real assassination, you can't convict people for killing imaginary people.
I'm not convinced that you looked at the article you linked.
> That’s because he was the Silk Road employee implicated in an elaborate, and fake, murder-for-hire scheme, created in part by a corrupt Drug Enforcement Administration (DEA) agent.
>DPR contacted one of his trusted drug dealer contacts, Nob, and asked him to kill Green for $40,000. Shortly after, Nob sent DPR photos of Green covered in Campbell’s Chicken & Stars soup and victim of an apparent asphyxiation, to prove the murder had been carried out.
> Unbeknown to DPR, Nob was no drug dealer. In fact, Nob was Carl Mark Force IV, the very same DEA agent who had arrested Green.
Both were fake. One was a con by the DEA and the other one a con by a single guy posing as executioner, victim and a slew of other colorful characters.
