People who are opposed to the death penalty (on spiritual grounds) for murder clearly fail to understand the importance and value that God places on Human life. God made man in His image. That is why he instituted the death penalty for murdering humans.
“Whoever sheds the blood of man, by man shall his blood be shed, for God made man in his own image. Gen 9:6 ESV
The life of one man is so important that when one takes an innocent life, one loses the right to continue to live. God has placed this judgment in the hands of men. It is the duty of a just society to carry it out.
And if society doesn't follow through, then it has declared 'open season' on innocent life. Witness gun deaths in black communities. Witness ISIS.