[slopinthebag]

This is a distinction without meaning. It makes no moral difference who dispenses justice, if said justice is justified.

[GMoromisato]

Except in our system of government, it is the government (via the courts) that decides what is "justified". It is literally called the "Judicial System".

You can't just decide on your own that violence is justified.

[pocksuppet]

You can, but the government really doesn't like having its authority disobeyed, and there will be consequences.