The government has a monopoly on the use of force. They are the only entity allowed to rob you.
On a side note, I believe Iowa's castle doctrine even extends so far as to even protect an individual who uses lethal force against home invaders with badges - of course only in instances where police invade a home unlawfully. For example, this applies in instances where police executing a no-knock warrant fail to check house numbers before breaking down a door, and inadvertently invade a house next door to the one specified in their warrant.
Wow, I wonder if there's anyone that actually shot the police for invading their home. Then again, what about the government robbing us via the banking system. What's the recourse?
Let's shoot the government?