The definition of justice is very broad. Justice a few hundred years ago can't be interpreted as the justice we have today.

We live in a world where every country has its own way of defining justice, which you are to follow, where you can't / are not allowed to take justice in your own hands.
Doing so is forbidden in islam since you would break the rules in the system you promised to follow, since we have abliged to live under those rules which have been set.

Someone correct me if I'm wrong