A simple form of practicing justice is to treat people in the way they deserve. Therefore some law enforcement appears to be vengeful although that's just one aspect of justice system.
The example of attempted crime and a committed one, provided by 'bananaunicorn4', grounds my opinion. Attempted crime need no punishment if the system stands for vengeance; nobody had really hurt. But, to make people share the morality the person is punished. And of course the committed crime punished with vengeance too.
The definition of justice is "just behavior or treatment", while the definition of vengeance is "punishment inflicted for an injury or wrong". If you were to ask me, the definition of vengeance sounds more like the "justice system". For evidence, just think about the law for attempted murder. It is the same as murder, with the exception of actually murdering. The people who commit both murder and attempted murder both have the desire of murder, so why not be faced with the same punishment? Because the "justice" system is vengeful. And look at the crime of manslaughter. The person did not wish to kill, it was merely accidental, so why should they be punished if it is truly justice? Because it is not justice, but brutish vengeance. The "justice" system is unjust.