The following is from R’ Moshe Sternbuch in his sefer Ta’am V’daas
?? ????? ????-?? ??????, ??????, ??? ??-???, ??????; ????? ???, ?????
1. Why do the eidim need to do the killing?
2. What does it mean at the end of the passuk ????? ???, ??????
Everything we see and hear has an effect on us. These eidim who saw such a horrible aveira being done (e.g. chillul shabbos)it too will have a horrible effect on them. Therefore, it is THESE EIDIM who require ????? ???, ?????, namely, getting this witnessed sin out of their system. Therefore the Torah says it is these witnesses who must carry out the initial stage of punishment.
One who lives in an area with chillul shabbos occurring must take proper precautions to ensure that they do not become immune to it. R’ Sternbuch suggests viewing such people as goyim, lest one come to question why Hashem allows people to be m’challel shabbos without punishing them. (note: he is not saying we should treat them like goyim, but rather that we should consider it like a goy is driving when we see them driving on Shabbos).