A better translation of 'thou shalt not kill' is 'thou shalt not murder' - a commandment in the second group of five commandments as to how people were to relate to each other on a personal level (the first group of five having to do with how people were to relate to God). As God's chosen people, the Children of Israel were given many other instructions and commandments as God instructed (primarily speaking to their leaders and the prophets) - the instruction of 'thou shalt not suffer a witch to live' was not something that an individual took to heart and acted on at a personal level. It was something that was acted on by the leadership or court of the time. As far as your question about whether you should obey the New Testament or the Old Testament, the answer is both. It's a very complicated question though and I don't know how to go at it without taking a lot of time since there is much background information needed to do justice to the question. If I find a link for an article that addresses this, I'll send it because I'm afraid I don't have the time right now.
So God's chosen people were assigned special rights?
I see. So there exists a better translation of this infallible document than what is written in it? So...I presume I may interpret the rest of the commandments in the same light? I may not personally covet, but it's fine for my nation to covet it's neighbor's, or my neighbor's property? I may not personally commit adultery, but it's perfectly ok for the state to commit adultery? I may not personally worship golden calfs, but if the state wants to, that's just fine?
Let me just compliment you on the finely tuned, flexible nature of biblical infallibiltiy.
Suppose my wife and I feel an urge to attend a wife-swapping party. Should I obey the adultary commandment, or the Golden Rule?
It's a very complicated question though and I don't know how to go at it without taking a lot of time
I don't doubt it.