The US Constitution creates a duty for the federal government to protect civil rights†, so, it's kinda crazy that there are hundreds of thousands of people who can kill people and deprive citizens of their rights who aren't licensed by the federal government.
The federal government should be supportive of states granting licenses as they see fit with a couple obvious caveats:
- pass an exam on civil liberties and civil rights
- not be part of a hate group
- declare past membership in any hate groups
- declare known associates who are part of hate groups
The point of licensing is not so much to make it hard to get a license to arrest people or carry a firearm as part of one's professional duties but to create an avenue for the federal government to pull licenses for police, deputy sheriffs, private security guards, etc. who misbehave.
My suspicion is that there would be some improvement immediately as cops weigh a bad shooting against losing a paycheck.
Overtime, the federal government will develop a set of rules that will bring police misconduct and police shootings down.
† Amendments I, II, IV, V, VI, VII, VIII, XIII, XIV, XV, XIX, XXIV & XXVI.