You seem to be saying that the reason he wasn't charged/convicted is because nobody was actually murdered. That seems hard for me to believe because the state would have known nobody was actually murdered when they were writing the indictment (which alleges the murder for hire plot) or when they were bringing the original charges.
It's also true that agents involved in the plot, the agent Ulbricht thought he was buying the hit from specifically, were arrested and convicted for stealing from the silk road. I think it's way more plausible that the government decided not to go in to the extent of the corruption of their officers at trial, because it would provide a good reason to think about what other government dishonesty their might be, rather than the government deciding that they can't charge attempted murder for hire because nobody died.
https://www.wired.com/2015/03/dea-agent-charged-acting-paid-...