Isn't it he legal, morally right, and justifiable taking of a human life?
The scripture tells us that the blood of the innocent cries out to God from the ground it is spilled in. It also tells us that eventually when that Blood gets to a certain level God will wipe out a nation. It also tells us that taking the life of those who shed innocent blood is one way to get rid of the sin of shedding innocent blood in a nation.
We can either allow the shedding of blood till our society gets saturated and then we get judged from a just God; or, we can shed the blood of the murders and make them pay for the shedding of innocent blood. If we do not make them pay for that then we will all pay.
God says he hates the hands that shed innocent blood. We always try to be kinder or nicer than God. All we do when we go that route is to bring a worse thing on all of us.
We need to kill those that have murdered in our midst. They need to be punished for the shedding of blood by having their own blood shed. That is justice.