>Who decide what is just or unjust?
Everybody does. Everybody must, because delegating the decision in itself is merely an option of that decision. Whether you obey power, follow your own greed, defer to social norms, or carefully weigh your own values and the consequences of your actions for yourself, all people are moral agents whether or not they realize which path they are following.
You raise a reasonable objection; I'm not advocating murder or wanton lawlessness. But here's a whole category of illegitimate laws that should be disobeyed: prohibitions born out of moral panic. These laws don't restrict actual harm, whether immediate like violence or accreted like pollution. They restrict the mere chance of harm by denying individual agency, out of fear that it could possibly be used for harm by even the tiniest number of people.
That's why simply discussing these laws isn't enough. They originate in fear and blindness that can't be reasoned against. The mere word of the thing evokes a fear response that shuts down thought: "drugs", "guns", "homosexuality", "racial integration". Good people have to be willing to break the law to show the difference between the panic and the actual outcomes.
Societal progress doesn't come from people musing about what they think they'd like to try, if only it were legal. It comes from people who risk everything to say "This forbidden thing is actually good and we know because we've done it, and it's so important that even banning it won't stop us".