We are a civilised society with resources and values.
I am not prepared for my own values to be dictated by those who are evil, uncivilised, wicked, selfish and who have no regard for human life or quality of life,
If I kill, or allow the state to kill on my behalf as a reaction to their crimes I am allowing their behaviour to dictate my actions.Tneir behaviour will have turned me into a killer.
Killing people is wrong. So wrong that killers do not deserve the privilege of freedom in a civilised society. They can be locked up for life to keep that freedom from them and to ensure they cannot kill or harm again.
And that enables us to keep our dignity and values in a civilised society.