So why is this any worse than Obama commuting Bradley Manning's sentence?
Manning had already spent a significant time behind bars. And, as a transexual, their incarceration was a greater hardship for them than it would have been otherwise.
On the other hand, Arpaio will not be spending one day behind bars as a result of Trump's pardon.
Then there is also the nature of the offences... Manning did reveal state secrets, but Arpaio violated the constitutional rights of its citizens, and he did so from a position of authority.
Lastly, while Arpaio was "only" found guilty of contempt of court (for ignoring a court order), there have been a mountain of other transgressions... from keeping prisoners in inhumane conditions, failure in handling cases of sexual abuse, and misuse of government funds. (The government has had to pay millions because of lawsuits caused by Arpaio.) To a competent person, Arpaio's track record should have been enough to realize that he was not worthy of a pardon.