Damouth,
To answer your analogy is very simple: if someone is attacking your familly after killing all the neighbors, it is perfectly ok to kill him, this is called self defense.
Now, if someone owns lots of guns, and has killed lots of people 10 years ago and scares the hell out of you...well, if you grab your shotgun and go kill him, you would most likely end up in jail. Now, how much you would get depends on your lawyer and you bank account
Aurel