You asked, “How did God treat official executioners when he says 'thou shall not kill'?”
Exodus 20:13 says that you must not murder, not that you must not kill.
To kill is “to cause the death of”
To murder is “the unlawful premeditated killing of one human being by another”
An official executioner is not being unlawful. Nor are they choosing the person to be put to death, if they are doing their job right. So, they are not planning to end the life of a particular person in an unlawful manner in a premeditated fashion. They are doing their job.
