I am sorry but this is just a stretch. It is easily disproven by knowing a few good cops. But the logic is also flawed. It assumes some sort of total understanding of and ascent to, a body of law known to be evil. You asserting that the law is evil because it doesn't comport to your worldview isn't enough. Having a law on the books that is evil but of which the officer was unaware doesn't prove they are not good.
Finally most oaths also include a clause to protect the rights, lives, and property of individuals. I presume you would call a law unjust precisely because it violated the citizen's Rights, lives, or property.
An officer could be completely justified in taking the oath and dispensing their duties in a manner that showed higher priority to higher goods when the details of their oath lead to a contradiction.