I can't say I know every detail of the case but I don't recall anyone getting killed or even hurt by Mr. Ulbricht so in my mind the punishment does not fit the crime. IMHO the death penalty should be off the table completely (go Nebraska!) and life in prison reserved for only violent offenders. You can argue that he enabled people to harm themselves but I think that's stretching it. If people want to take drugs, even take too much drugs their going to get it somewhere. If drugs were legal and treatment of abuse the focus instead of punishment Silk Road wouldn't have existed in the first place.
It's interesting to note that even if one of these overdose victims had health problems that predisposed them to overdose mortality --- something that is pretty far from being established --- there is actually a legal rule that contemplates this circumstance directly: google "the eggshell skull rule". The prosecution memo invokes the rule.