We understand it some. It is very, very rare for a judge to join the legislative or executive branches. I can't think of an example at all.
However, people moving between legislative and executive branches of the government is pretty common. State legislators become governors, governors become federal legislators, federal legislators run for president.
In the US, prosecutors are part of the executive branch. That's because their job is to represent the state. They are supposed to seek justice, but in the context of an adversarial system. Judges are nominally neutral umpires; prosecutors battle with defense attorneys. Both sorts of attorneys are expected to fight vigorously but fairly for their side. So the burden of fairness mainly falls on the system and the judges, not the partisans on either side.
But yes, that does produce a conflict of interest when deciding what crimes to prosecute. They have an incentive to pick cases that benefit both the state and themselves.
However, people moving between legislative and executive branches of the government is pretty common. State legislators become governors, governors become federal legislators, federal legislators run for president.
In the US, prosecutors are part of the executive branch. That's because their job is to represent the state. They are supposed to seek justice, but in the context of an adversarial system. Judges are nominally neutral umpires; prosecutors battle with defense attorneys. Both sorts of attorneys are expected to fight vigorously but fairly for their side. So the burden of fairness mainly falls on the system and the judges, not the partisans on either side.
But yes, that does produce a conflict of interest when deciding what crimes to prosecute. They have an incentive to pick cases that benefit both the state and themselves.