Arpaio was to enforce the law, NOT to determine the law or make his own law!  He broke the law and was punished by the legal system.  The COURTS determine what the law is and how to punish evil doers that ignore and violate the laws.   That is what Arpaio did when he ignored a court order to stop violating the laws limiting his power to violate the constitutions.  Then he also committed Contempt of court (another illegal act) by continuing to violate court orders.
Trumps pardon encourages other militaristic police to violate laws because the president says it's okay.  This is very dangerous to a free people when martial law is not subject to democratic controls nor to the Courts.  When that happens, you no longer have democracy, but you have despotism.