At first, I thought that the President's pardon of Sheriff Joe Arpaio was bad, but wasn't unusually bad. This is not true. The closest I could find was George W. Bush's commutation of the sentences of former Border Patrol agents Ignacio Ramos and Jose Compean who shot a Mexican drug dealer and tried to cover it up, but that offence had its roots in the challenge of peace officers' having to make split-second decisions-- I don't agree with W.'s choice, but I understand that that is a matter about which reasonable people might disagree.
Joe Arpaio had been fighting a racial-profiling case that has been going against his Maricopa County Sheriff's Office on for years. Before the case went to trial, U.S. District Court Judge G. Murray Snow, a George W. Bush appointee, ordered the agency in 2011 to stop detaining people solely on suspicion they were in the country illegally. They were either to arrest individuals for a state crime or let them be on their way. In 2013, Snow found deputies had used race as a factor in their policing, and ordered sweeping reforms of the office’s policies. But Arpaio’s deputies continued business as usual for at least 17 more months. According to trial testimony, 171 people were illegally detained by MCSO deputies and turned over to federal immigration authorities. Snow found Arpaio in civil contempt for violating his order, and forwarded the case for criminal contempt. Prosecutors from the U.S Department of Justice’s Public Integrity Unit prosecuted the case in a bench trial this summer before U.S. District Court Judge Susan Bolton, a senior district judge appointed by Bill Clinton. Arpaio’s attorneys argued the violations were unintentional, and that Arpaio delegated the court’s order to subordinates. Bolton flatly rejected that argument. In her ruling, she said evidence showed Arpaio’s “flagrant disregard” for Snow’s order. Arpaio was scheduled to be sentenced for the contempt-of-court charge on October 5th, and he could have served up to six months in prison.
The President disagreed with Judge Bolton's ruling. There is no reason to think that he thought her ruling inaccurate.
Second, pardoning contempt of court is particularly sensitive; the contempt power is a vital tool for keeping order in courts, and the President's action encourages other popular bad actors not to obey federal judges, magistrates, and marshals e.g. Roy Moore. The executive branch disempowers the judicial branch.
Third, the President is being investigated. By this action, he signals the other targets of investigation that he can use the pardon power to offer better deals to them than Robert Mueller, among other people, can.
Texas has faced similar problems in the past, and we have changed our laws such that the governor doesn't have pardon powers. They have been delegated to a Board of Pardons and Paroles-- now the Board is chosen by the Governor with the advice and consent of the Senate, but it means that if there is an abuse of the clemency powers it would need to be authorized by a majority of the Board and the Governor, which can either be sanctioned by criminal laws, or, in the Governor's case, by impeachment.
It may be easy to impeach a president for corrupt use of the pardon power or hard, but the president's pardon of Arpaio is a dereliction of duty, an attack on the separation of powers, and a corrupt signal to criminal suspects that they need not obey the law.