The separation of legislation - or arbitration; the two are in the same bag for this purpose - and enforcement is often seen as a good thing, especially by those who defend the American constitution, but it actually is the source of horrible incentive problems.
In a society where the roles of legislation and enforcement are systemically separated, police are able to excuse their acts of violence carrying out unjust laws by telling themselves "I don't make the rules, I just enforce them", while politicians are able to excuse their acts of ordering the police to commit violence against us because they're not personally seeing the cruelty entailed by their "laws".
Politicians don't have to look at the innocent people being dragged out of their homes, away from their friends and jobs, and put in cages for no objectively immoral act. They don't have to see the people's lives they ruin. They don't have to see the pain and the righteous anger they cause. And when this results in mass shooters, the politicians don't have to confront the reality that they are responsible.
The separation of legislation and enforcement is one of the most vile and essential ideas powering oppression, and I have no doubt that the pejorative phrase "judge, jury, and executioner" was created to reinforce this idea.