Right to live

Everyone has an inalienable human right to life, even those who commit murder; sentencing a person to death and executing them violates that right.

This is very similar to the 'value of life' argument, but approached from the perspective of human rights.

The counter-argument is that a person can, by their actions, forfeit human rights, and that murderers forfeit their right to life.

Another example will make this clear - a person forfeits their right to life if they start a murderous attack and the only way the victim can save their own life is by killing the attacker.