Two lessons from this case. First, don't threaten to kill a federal judge. Second, don't ask a fellow inmate to arrange the hit. Because he'll almost unquestionably roll on to you. And the FBI cares. Duh.
Robert Stewart didn't learn those lessons, and he'll spend an extra 20 years in jail as a result. Even though he was in jail and in utterly in no position to carry out his threat. 20 years. That's a lot of time. 20 years to think about your abject idiocy in electing to engage in a-bit-more-than-idle-speculation about killing the judge who sentenced you. Hardly worth it.
P.S. - How psyched was Stewart's inmate "friend" when Stewart first hypothesized about killing the judge? You just know the guy must have almost peed his pants at the suggestion. Because you know -- know -- that you're going to earn from being a snitch on that one. Just know it. And earn lots.