I think the point is that justice shouldn't be arbitrary like that.
If you are convicted of certain crimes, your sentence -- even within the range appropriate for those crimes -- should not take into account other crimes that the person was suspected of committing, but it couldn't be proven beyond a reasonable doubt.
That's the entire point of evidentiary standards: in the first place, it's to avoid putting people in jail at all if we can't prove they did it to a particular level of confidence. But we should also not be inflating sentences (again, even if the end result is still within the guided range for the convicted crimes) just because we think they did other, worse things too.
Maybe Ulbrict did deserve a much longer sentence. But the prosecution apparently could not prove the more serious crimes. His sentence should not be influenced by things the prosecution could not prove. I'm sure there are other defendants who were convicted of crimes, but suspected of worse crimes that they didn't actually commit in reality. Maybe Ulbricht got what he deserved, but these other people got worse than they deserved.
> Do they not think that hiring a contract killer is, like, no big deal?
To sum up: what people think about this is entirely irrelevant. He was not convicted for hiring a contract killer, so that should have zero bearing on his sentence. Even if his sentence was in the range appropriate for what he was convicted for, it's a range, and if his sentence was pushed higher in that range because of other things the judge believed he did, that's not justice.