The presidential oath enjoins the new president to swear or affirm that he "will to the best of his ability, preserve, protect and defend the Constitution of the United States."
By pardoning Joe Arpaio, Trump has violated his oath of office. Arpaio was found guilty of violating the constitutional rights of American citizens, and charged with contempt of court for continuing to do so in the face of a lawful order from the court. In pardoning Arpaio, Trump has indicated that he is willing to pardon, not just those who are convicted of commiting criminal acts and show appropriate remorse, but even those who fundamentally violate the Constitution of the United States. In so doing, he also shows contempt for the legal process embodied in the Bill of Rights, and hints again at his willingness to undermine the special prosecutor Robert Mueller.
Combined with his continued violation of the emoluments clause, it is apparent to me that articles of impeachment are justified and warranted.