He was akin to a one man cartel, why shouldn't he be locked up? I'd guess there is a high risk he'd do it again if he had the chance. Unless you think it was OK because he was 26 years old and first time offender...
I vaguely recall reading he (unsuccessfully) paid to assassinate people.
Edit:
Read it here https://www.wired.com/2015/04/silk-road-1/
Later that day, DPR messaged Nob.
DREAD: ok, so can you change the order to execute rather than torture?
DREAD: he was on the inside for a while, and now that he’s been arrested, I’m afraid he’ll give up info.