As I answered here:
Rather than command us to put witches to death (just like any other death penalty in the Pentateuch) the bible here instructs others (perhaps the courts) to not let any witches live, which seems to be a simple translation of this verse from Hebrew to english.
Rashbam notes that this must be a specific instruction given to others to "hunt them down", because witches tended to practice witchcraft in hiding. Shadal follows this understanding as well.
(Various other Jewish commentaries suggest that this language is used to teach us that the commandment is to kill witches immediately however possible, as opposed to waiting for judgment.)
Mecklenburg suggests that this wording is used as a parallel to Deuteronomy 20:16, which refers to the killing of all of the Canaanite Nations, and tells us that we should not have mercy, even on women. This applies here as well, where the verse is discussing witchcraft, which was generally performed by women.