We still have the command from God to kill all murderers.
That's Old Testament, right?
If I'm not mistaken, the OT also authorizes the death penalty for adultery, or even for children disobeying their parents.
Not every dogma from the Old Covenant still applies under the New Covenant, and that's as it should be. The Judaic Law has been superseded by the Love of Christ.
That doesn't mean that "Thou shalt not kill" is invalid. But the Old Covenant, with its stridently legalistic promulgation of rules governing every nuance of living, was insufficient. That's why we have a New Covenant.