Armchair policeman

[Justin Bieber]

Noun.
What a person becomes when, upon receiving word of a police incident [from the internet], the news, or word of mouth, one gives advice on how the police should have properly [handled] the situation in line with police [protocol], the law, etc.
Usually, this occurs when the police have been accused of wrongdoing or, even more commonly, when the police have been accused of no wrongdoing yet the receiving party believes they are almost entirely at fault.
Common occurrences include drug busts, traffic stops, and police shootings.