There was no 'Holocaust flag'. I wonder if you are confusing the Holocaust with something else?
It is the Nazi symbol, the swastika.
The symbol of David in the flag was the same as that which the Nazis made the Jews in Europe wear on their clothing as a badge of Jewish identity.
it is not, it was created before the modern state of Israel and looks remarkably similar.
Yes. A great number of disgusting, distasteful and unpleasant things are legal in the United States, of which the Nazi flag, the ISIS flag and the Confederate flag are only a few. We have this peculiar custom called "Freedom of Speech", enshrined in the First Amendment to our Constitution. Are you offended by that? Then don't look.
The Holocaust
To remember the Holocaust.
The Holocaust is a concept not a person.
The Holocaust wasn't a war. Please see the related questions.
One aspect of the Holocaust is murder, there was a lot of killing in the Holocaust.
The Holocaust started in Europe.
There were no elections in Germany in the Holocaust.