Perhaps, but institutions by nature of being well known have more public accountability. If, say, the police began giving out harsh punishments for personal reasons, the public would become restless. But an individual, or even a small group, could fly under the radar and get away with whatever they wanted.

For example, I've heard of a group of likely student vigilantes calling themselves the Phantom Thieves. Because they operate independently, nobody even knows who they are. They dole out their brand of justice, but nothing would stop them from targeting people for personal reasons entirely divorced from the idea of punishing criminals.