Is this not a general instruction from God that murderers should be put to death?
Gen. 9: 6 “Whoever sheds man’s blood,By man his blood shall be shed;
For in the image of God He made man.
Perfect time for Jesus to clarify the death penalty issue here, but didn't change anything:
Matt 15:3 He answered and said to them, “Why do you also transgress the commandment of God because of your tradition? 4 For God commanded, saying, ‘Honor your father and your mother’; and, ‘He who curses father or mother, let him be put to death.’
Paul didn't refute the government's ability to carry out capital punishment:
Acts 25:10 So Paul said, “I stand at Caesar’s judgment seat, where I ought to be judged. To the Jews I have done no wrong, as you very well know. 11 For if I am an offender, or have committed anything deserving of death, I do not object to dying;
In fact, Paul affirmed it:
Romans 13: 3 For rulers are not a terror to good works, but to evil. Do you want to be unafraid of the authority? Do what is good, and you will have praise from the same. 4 For he is God’s minister to you for good. But if you do evil, be afraid; for he does not bear the sword in vain; for he is God’s minister, an avenger to execute wrath on him who practices evil.