It depends on the judicial system. I'm guessing you're from an Earth, probably the United States? Where, yes, bounty hunters are tasked with bringing defendants in to stand trial. In the Galactic Federation, it's possible to conduct a trial in the absence of the defendant. The standard for proof is set higher for the prosecutor than it would be with the defendant on hand, and a counsel pro tem is assigned to make the defense case on procedural grounds. At any point, even after the verdict is rendered, a defendant can turn themselves in and get a continuation of the trial where they get to present their side, but if they don't avail themselves of their judicial rights, then they're stuck with the verdict they've got, and depending on the severity of the crime, bounty hunters may be empowered to deliver the sentence.