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.
The medieval philosopher and theologian Thomas Aquinas made this point very clearly:
Therefore if any man is dangerous to the community and is subverting it by some sin, the treatment to be commended is his execution in order to preserve the common good... Therefore to kill a man who retains his natural worthiness is intrinsically evil, although it may be justifiable to kill a sinner just as it is to kill a beast, for, as Aristotle points out, an evil man is worse than a beast and more harmful.
Thomas Aquinas, Summa theologiae
Aquinas is saying that certain contexts change a bad act (killing) into a good act (killing to repair the violation of justice done by the person killed, and killing a person who has forfeited their natural worthiness by killing).