Is the world off better with or without the police ?
It is better off with police, but certainly, the things they are critiqued for go against the very laws established. A cop that shoots a black guy 10 times in the back out of racist agenda, will face consequences. Saying that the world is better off with police and absolute justice requires a black kid stealing bubble gum being executed is quite wild, you know what I mean? Same goes for Akainu blowing up a ship with hundreds of civilians because there might be a scholar who dared to study the past (as if latter is a crime to begin with, justifying to annihilate an island). Akainu by our standards would be a war criminal.
Dragon did it right leaving as he figured there isn't really justice in there. The Marine's main job is to protect the interests of the ruling class, which commits genocides and enslaves people. Arguing that the institution Marines also handles a lot of bad guys can't be used as a counter argument.