Our system of government is based upon the balance of powers among its three branches: executive, legislative and judicial. While the president has unlimited power to pardon people except for impeachment, the recent decision to pardon Sheriff Joe Arpaio undermines the rule of law. The sheriff has abused his authority and broken the law many times himself. He has denied citizens their constitutional rights, inflicted cruel and unusual punishment on hundreds of prisoners, and illegally arrested reporters covering him. He was convicted of contempt for refusing to obey a Federal judge’s order to stop illegally detaining people on suspicion of being undocumented immigrants.
In trying to justify the pardon the president endorsed the use of these illegal tactics as a way to secure the border and gave tacit approval to the sheriff’s illegal actions. This abuse of executive power to defy the judicial branch undermines the rule of law. I think the president should be removed from office for that, and before he does more damage in this regard.
Addendum: Law professor Sheila Kennedy explains this very well in a recent blog post http://www.sheilakennedy.net/2017/08/pardon-me/