From the linked documents, straight from the judge as sentencing adjustments:

    Ulbricht's directed violence here is and 
    relates to the murders for hire which he is alleged to 
    have commissioned and paid for. The Court must 
    determine whether these allegations have been
    demonstrated by a preponderance of the evidence and I 
    find that there is ample and unambiguous evidence that
    Ulbricht commissioned five murders as part of his 
    efforts to protect his criminal enterprise and that he paid
    for these murders. There is no evidence that he was 
    role-playing.

    The Court finds that the evidence is clear and
    unambiguous and it far exceeds the necessary
    preponderance findings, that Ulbricht believed he was
    paying for murders of those he wanted eliminated, and 
    that he believed they had in fact been murdered.

Check out pages 18 - 19.

http://www.scribd.com/doc/283722300/Ross-Ulbricht-Sentencing

then why didn't they convict him of it? Or more importantly, why didn't the prosecution keep the charge?

Notice indictment, first count, overt act B.

For the benefit of others, here is the indictment: http://www.justice.gov/sites/default/files/usao-sdny/legacy/...

The murder for hire thing is on page 5.

This is why I don't trust you, Tom. You know he wasn't convicted of it, and you agree through acquiescence. But knowing that, you pretend to conflate indictment and conviction, as if it justifies what I said being wrong.

That's lying, Tom, and it isn't right.