There is no obligation for police officers to enforce any law. Otherwise, how do you have separation of powers? If the executives have to enforce every law, and the judiciary has to prosecute every arrestee, then the legislature has all the power, and the other two have little or none.
As it is, the practice of "officer's discretion" and for that matter "prosecutorial discretion" are well known, meaning they can decline to arrest or prosecute, respectively, although these practices are frequently used to let other government agents off the hook, when a private citizen would feel the full force of law under the same circumstances.
Otherwise, how do you have separation of powers? If the executives have to enforce every law, and the judiciary has to prosecute every arrestee, then the legislature has all the power, and the other two have little or none.
I don't see that that follows.
Does the judiciary prosecute? Does the judiciary HAVE to prosecute every arrestee? Who says and since when? We have grand juries and preliminary hearings, after all.
A bust's not being prosecuted or a prosecution ending up in acquittal doesn't mean it was a bad bust or a bad prosecution. Sure we don't want things like that to happen, it's not optimal. But a good arrest is one that can be shown to be based on probable cause, not on certainty of guilt.
My answer was, I don't think an officer is obliged to enforce any law, which I guess makes you think I didn't answer your question. But, supposing there was such an obligation, I would say yes, the officer should resign.
"Just following orders" didn't do it for Nazi officers after WWII, and rightly so - they should have resigned, rather than following those orders. In order to prevent another Nazi Germany from ever being possible again, one has to have the option of not following orders. (The present UCMJ provides this out: a soldier has the duty to obey lawful orders.)