But then you need a judge that also wants to destroy the realm. Underground membership is secret so he has to ban everyone. It takes three days for the ban to take effect, which potentially gives you enough time to lead a successful rebellion.
On top of that if your realm has multiple duchies dukes can secede and regions can change loyalty to the new realm. Then that realm can declare war on the original, oust the mad king and his judge and rename the realm back to its original name.
Like Tim said, realms tend to have more safeguards because they are viewed as the most important grouping. But they are not immune. What if you are a single duchy realm, the one in question was on FEI. What if nearly every noble is away on a war so Lords and Dukes are too far away to take much action?
Of course once you have banned almost everyone then what, you potentially don't have the nobles to even have a Lord for every region, but if your goal is to destroy or severely hurt the realm what do you care? Even if you have a structure that can split off and fight against the mad King, if you already have conflicts going, and those opponents decide to press the advantage what then?
