I have had that routine for nearly 10 years now. It is about the misunderstanding and ignorance of what is clearly a tragic and horrific situation. My comic persona is that of a man who speaks with great arrogance and authority but who along the way reveals his immense stupidity.
In this particular routine, I envisage an almost slapstick version of the Nazis entering the home of Anne Frank on a daily basis and always failing to bother to "look upstairs".
I even have one of them suggest, "Looking upstairs today, Sarge?" The officer replies, "No, let's move on."
The first Nazi then says: "What's that tapping sound?" - as I mime using an old fashioned typewriter. Again the joke here is the supremely stupid assumption that Anne Frank obliviously and noisily typed her diary.
The Sarge (who I am portraying as a lazy and incompetent Nazi) answers, "Mice! Move on".
The final layer of ignorance in the routine is that, instead of taking the obvious and correct stance that Nazis were disgusting, immoral and evil, I merely conclude that they were "rubbish" because of their inability to find Anne Frank earlier - like it was all part of a big, mutually agreed game of hide-and-seek.
I can see if you took this routine at face value as my real opinion on this profound and heroic tragedy, it could be deemed highly offensive. However, this is obviously an absurd comic position with the audience well in on the joke, fully aware that I am saying the exact opposite of what every right-minded person thinks.
I often get accused of finding comedy in places where no comedy is to be found. I feel you can make a joke about anything. It just depends on what the joke is. Comedy comes from a good or a bad place and the problem is in its interpretation, with some people confusing the subject of a joke with the joke's real target. The target of this joke is the comedian's ignorance.