While pirates take the main stage in One Piece, one of the recurring antagonists for the Straw Hats is the Marines. The Marines serve the World Government and maintain the order of things as they currently are, for both better and worse. Despite their status as keepers of the peace, they also suffer from deep corruption -- as the higher-ups are some of the strongest people in the world, they're one of the most powerful groups of the story.
The Marines are often depicted as being horrible people, taking advantage of those who rely on them for safety. The Marines take this reliance for granted and abuse the towns they are stationed in. While not every single Marine is a horrible human being, the vast majority of them are. The most susceptible to corruption are those closest to the nobility of the World Government. Many times, they abuse not only citizens but also their own men. Having to go up against pirates in a constant bid to keep the order of things, these are some of the toughest fighters in the world.
One of the biggest examples of this corruption is Admiral Akainu. Rumored to be the most hated character in the entire series, he is a prime example of power going to one's head and not understanding the true meaning of justice. Instead, he follows the doctrine of Absolute Justice, meaning that he will punish anyone caught breaking the law, no matter the circumstances. Many Marines follow this doctrine, thinking they are the only force of good in the world. There are no shades of gray to them, and pirates -- along with anyone who helps them -- must be destroyed at all costs.
The leaders of the World Government do nothing to stop this mentality and abuse, as it helps them stay in power. Without a system of checks and balances, the two corrupt forces oppress the people they are supposed to lead and protect. The higher-ups only gain their positions when the leaders deem it so, regardless of whether the promotion is truly earned. They only seek to promote those who will keep their own corrupt laws in place. Because of this, the members of the Marines who are given positions of power abuse it, aware that nobody will stop them.
With such a corrupt system in place, it's no wonder the Marines are so hated among the common citizens. While there are people like Koby who are trying to change the system from within, it's hard when even the world leaders refuse to do anything to quell the abuse. Although some want the Marines to be a force for good and change, that dream seems very far off given the current state of affairs. The townsfolk can't do much against them because several of the admirals are Devil Fruit users, genetically enhanced or just unbelievably strong -- the average person therefore isn't likely to be able to take them on. However, with Luffy so close to defeating Kaido and Big Mom, change may eventually be on the horizon.