What do you do if a murderer breaks in your house?

Keep the door of the room you are in closed - lock it or if you don't have a lock, put something against it. Dial 999 or 911. If you don't have a phone then quickly run to the window and scream or shout for help.

Added: If you own a firearm - arm yourself and announce the fact to the intruder. If they try to break in and attack you - use it! That's what your rights under the Second Amendment are all about - self protection.