He wasn't charged with that, although all reports indicate he did do it. All sorts of conspiracy theories as to why he wasn't charged, most likely boils down to LE or prosecutorial incompetence.
It was, in fact, pay of the allegations of the crimes he was charged with, and it explicitly was a sentencing factor. He wasn't charged with, say, attempted murder or conspiracy to commit murder for the alleged murder-for-hire scheme, though.
> Federal prosecutors alleged that Ulbricht had paid $730,000 in murder-for-hire deals targeting at least five people,[29] allegedly because they threatened to reveal Ulbricht's Silk Road enterprise.[37][38] Prosecutors believe no contracted killing actually occurred.[29] Ulbricht was not charged in his trial in New York federal court with any murder-for-hire,[29][39] but evidence was introduced at trial supporting the allegations.[29][38] The evidence that Ulbricht had commissioned murders was considered by the judge in sentencing Ulbricht to life, and was a factor in the Second Circuit's decision to affirm the life sentence.