Fifth Commandment

When was "Thou shall not commit murder" changed to "Thou shall not kill"?

I am not sure which source you are referencing for this change of wording. The Hebrew word in the Fifth Commandment (Exodus 20:13 and Deuteronomy 5:17) prohibits murder—the illegal and inappropriate taking of human life—and does not forbid all taking of life. So, for example, governments can rightly implement capital punishment (Romans 13:4) if they so choose.