About these purple "no trespassing" markings, can't this be used by someone to get away with murder? You drag someone inside the fence, you shoot him dead, and then claim he was trespassing and did not respond to verbal warnings and warning shots. How would the authorities in the relevant USA territories deal with this?

In contrast to the above, in many (most?) countries in the world, you will be put to prison if you shoot a burglar when he is inside your house, even if you legally carry a weapon. Even more surprising is the fact that, in the UK, police officers are not normally allowed to carry firearms (IIRC -- UK WOers, please confirm if that's the case).