The civilized world organizes and bands together to protect itself
from threats that singularly the individual citizens could not
handle alone.  In many cases, that threat is you.  

If you insist on killing guards and shopkeepers, be prepared to
have the entire justice system on that planet mobilized to attack
you on sight.  You won't be able to sell to shopkeepers on that
planet, you won't be able to step foot in any city without
being attacked by guards, and you will be continually harassed
until you leave. 

Upon being killed by a guard or shopkeeper, you will be less
wanted.  Upon killing a guard or shopkeeper, you will be
more wanted.  It's literally a corpse for a corpse.  

One can be pardoned by a Sentinel to remove this terrible
record (HELP SKILL PARDON), but then again, a Sentinel can
also put you on this very same list (HELP SKILL WARRANT).

In conclusion, don't kill guards or shopkeepers if you ever
expect to visit other places.  Remember, you can end up
dragging your friends down with you as well -- whoever
gets the killing blow gets the lawbreaker code.  A mage
who is part of a party which gets attacked by guards because
the fighter is a lawbreaker will cause the mage to become
a lawbreaker as well if he gets the killing blow on that 
guard.

NOTE:  Partying with lawbreakers is a good way to become 
known as a lawbreaker yourself -- if your party members kill
a guard or merchant while you're in their party, word will get
out that you run with a rough crowd -- and you'll be wanted too!