The purpose of the police is to keep the peace & to protect & serve the RIGHTS of the citizens ,the purpose of the military is to KILL the ENEMIES OF THE NATION STATE & break their stuff .
When you try to use the military as police or militarize the police the citizenry become rightly or wrongly “ENEMIES OF THE STATE”. This is not a good thing as it leads to the viewing of any protest as attacks on the state & that any amount of violence is then justifies in response.
Law enforcement meant exactly that: enforcing the application of the law, not protecting people. The role of the sheriff in rural America wasn't to patrol his jurisdiction and protect the people who lived there. It was to apprehend an accused criminal and protect said criminal from an armed citizenry who were hell-bent on exacting their own justice before the accused criminal was convicted in a court of law. In other words, the whole social order was built on the assumption that the "civilians" were at least as well armed as the "police" -- and certainly outnumbered them.