Do they have to uphold the rule of law?

When a law is passed it is the law.  Right?  Those that violate it are in danger of prosecution from those who enforce it.  If they enforce it.  Is it really optional for the enforcers to enforce the law?  Can one take an oath to uphold the rule of law and then tell its violators not to worry about the law because they want to change it in the future?  Can one choose not enforce a law because they need it to bribe others with?  Impeachment comes to my mind, but I am an anti-government type to begin with. What do you think?