The idea that God gives "us" the right to judge each other and punish is inaccurate. If we are catholic chrisitians, then we follow Christ (christ-ian, remember?). The revelation of Christ for catholics is revealed via two fountains supplied under the guidance and inspiration of the Holy Spirit--#1 Sacred Scripture, which refers to the bible, and #2 Sacred Tradition, which refers to the body of the Church’s pronouncements and writings, especially of the early fathers, and particularly of the teachings declared infallible by the Pope.
Hence, if I kill somebody, I commit murder and violate both Sacred Scripture (5th commandment-Exodus 2-13)and Church teaching (see Catechism of the Catholic Church, 2258). But "Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm." [Cath Catech, #2265).
In the arena of nations, similar principles apply: a nation’s political representative may indeed "judge" or I prefer evaluate that another country to determine if a "Just War" against that other country is warranted. As a catholic I would want to my President to do this carefully and not err, because Sacred Tradition (see Vaticans site) tells me that if certain conditions apply there is just war. If not the nation [but not our honest troops who trust their erring President] are engaging in murder.