When is it okay to attack the police? 

This is not a question of whether or not it's a good idea, but under what circumstances it becomes 'okay' within the common framework of libertarian anarchism (AKA voluntaryism, free market anarchism, etc.)

I am conflicted on the subject and it troubles me - allow me to explain.  If you're reading this, you're likely as aware as I am of the atrocities carried out by law enforcement every day in the United States.  They quite clearly commit, in the scope of their duties, rights violations ranging from confiscation of property, to denial of bodily freedom, to bodily injury and disfigurement.  Too often, they use deadly force against unarmed victims.  In a vast majority of these cases, the confrontation was initiated by them, in the name of non-crimes.  All of this, taken with the overwhelming prison population and the unspeakable acts carried out by the US government overseas, is very clearly organized crime writ large - I think one can rightfully view US law enforcement, from federal to municipal, as an occupying hostile force.

So why am I so revolted by the idea of attacking cops?  

I'd most certainly have no moral issue with fighting Russian, Chinese, or any other variety of foreign soldiers occupying my homeland.  I'd have no qualms with shooting Waffen SS or Gestapo dead in Europe during the second World War.  However, when it comes to American law enforcement, or even the hypothetical US soldiers in a martial law scenario, I become morally, or perhaps just emotionally, uncomfortable.  After Micah Xavier Johnson's ambush in Dallas, Texas where he killed five cops, I found myself not only groaning over the the practical implications, but also sad because the cops who were shot weren't the ones guilty of the most recent high profile cases of police brutality. They were very possibly "innocent" in my mind, and I still feel this way.  And that's where I caught myself - if you held all these things constant, and applied them to Nazis or foreign soldiers, my apprehension vanishes.

Why?  Help me understand if I'm being morally or intellectually dishonest, or holding misplaced emotions.  I am without a doubt making some sort of error here, and I desperately want to see it.