Rule 10

(RULE 10) No-PKs are not allowed to killsteal.  PKs can, if within range.
 
   Sometimes players feel the urge to jump into a fight and kill a mobile 
that is already fighting someone else.  If the fighter involved does not 
specifically ask for assistance, and is not in a group, the mobile belongs 
to the existing combatant.  Those proven to have committed this crime will
be dealt penalties based on the severity of the situation.  Mobiles in
PK fights are free game to any other PKs within level range.  No-PKs may 
not attack someone else's kill unless invited to do so.

(See also: CLAN, RULES, 'PK RULES'.)