"The judge locked the container."
Translation:La juĝisto ŝlosis la ujon.
-ulo relates to a person with that property (blindulo = blind person).
-isto relates to a person with that profession (or very regular occupation).
That's why your proposed words would not make too much sense. "juĝulo" would be a person with the property to be a verdict, "bonulo" a person with the profession to be good.
As I understand it, -isto indicates a specialist, while -ulo indicates your average joe. -isto is usually applied to some occupations, sports & hobbies, rather than adjectives. If a person specialised in being good, they'd be a bonisto, while a judge specialises in judgement, hence jugxisto. A jugxulo would be a regular judging guy (like that guy at work who's always criticising people, maybe?) (intermediate Esperanto, not fluent, not an expert, corrections by more experienced Esperantistoj welcomed)