He wasn't sentenced for attempted murder, the sentence Ulbricht received was within the range provided by statute for the crimes he was convicted of. Judges have discretion in sentencing and they are allowed to consider the character of the defendant. The fact that Ulbricht attempted to murder people was demonstrated to the judges satisfaction during the trial and influenced her to sentence at the higher end of the range allowed for the crimes he was duly convicted.

This cuts both ways as judges often adjust their sentencing downward based on mitigating evidence. For both aggravating and mitigating circumstances evidence does need to be submitted, and there are standards of proof to be applied. It's just that the procedural rules can be different and, depending on the context and jurisdiction, sufficiency can be decided by the judge alone. In some jurisdictions, for example, aggravating evidence may need to be put to the jury, while mitigating evidence need not be.

The U.S. is rather unique in providing a right to jury trials for most--in practice almost all, including misdemeanor--criminal cases. And this is a major factor for why sentencing is so harsh and prosecutions so slow in the U.S. In myriad ways the cost of criminal trials has induced the system to arrive at its current state favoring plea deals, with overlapping crimes and severe maximum penalties as cudgels. Be careful about what kind of "protections" you want to impose.

Sentencing is complicated in the US. Generally speaking, they have a huge range and a standard for computing where one falls in that range, but everything within that range is open to judge's discretion. Life without parole was within that range for the crimes that Ulbricht was convicted of.

This standard is an enormous document, https://www.ussc.gov/guidelines which lays out the rules for adjustments. Evidence is admissible (by both sides!) for sentencing, with a lower standard of evidence and burden of proof, to either raise or lower the sentence within the very wide numbers of what the conviction was for. So the Judge in this case found that the lower burden of proof was met for additional violent crimes being committed (with Ulbricht's legal team having an opportunity to rebut), and that impacts the sentencing calculations.

Not a lawyer, but I have listened to US lawyers on podcasts.

Other acts of those charged are routinely brought up in trials. Fir example, criminals being charged with crime A that already committed similar crimes in the past are used to show that the likelihood of crime A being committed this time is higher.