What crime did Arpaio commit, after all? He used his legal power to select people based on their ethnicity, and then place them in what he himself called concentration camps.
You may want to argue about exactly what a person needs to do to be classified on the same degraded level of humanity as Nazis, but this is certainly it. Arpaio then defied judicial rulings to stop this hateful behavior; it was for this that he was found guilty.
Trump, in pardoning Arpaio, declared openly, if not is so many words, that it is okay to place people in concentration camps because of their ethnic origin, regardless of what the law says.
That is Nazism pure and simple. That is why Trump's pardon represents an open embrace by the President of the United States of the most degraded form of government the world has ever known.
The fact that he still resides in the White House, after this incident is a horrible condemnation of his own party, which is the only agency with the power to remove him, and is a massive step toward a nation where imprisoning people in camps because of their ethnicity, or any other reason that suits the leaders' fantasy, will be an accepted practice.