Mishpatim: Ramban: Negative Influences: Mitzvah 62 – Concept 552
“You shall not permit a sorceress to live.” (Exodus 22:17) The court must not allow a sorcerer to live. (Rambam, Hilchot Sanhedrin – The Laws of the Sanhedrin) In connection with all those who are guilty of death, He has said above, “He shall surely be put to death,” meaning he is liable to death, and it is a positive commandment upon us to slay him, based upon the verse, which says, “And you shall put away the evil from your midst.” Or, it may be that this obligation on us is derived from the very expression’ Yumat,” he shall be put to death,” which He used in these cases.
But here, however, He did not say, “A sorceress shall be put to death,” but in this case He warned us in a stricter manner by means of a negative commandment, that we shall not suffer her to live.
The reason for this is that the sorceress is “defiled of name and full of tumult,” (Ezekiel 22:5) and fools are mislead by her, therefore He was more stringent and admonished us with a prohibition.
We find a similar severity in relation to all those who cause snares for many people, as in the case of a misleader after idols and in the case of a murderer. (Ramban)