The gemara says that we don't kill eidim zomemim if the punishment on the accused has already been carried out "kaasher zomam" - vi-lo kaasher osoh.
Fregt the velt: How come we don 't kill them for another reason - they are murderers for they carried out the death penalty as is the law for all witnesses. "Yad ho-eidim tihiyeh bo barishona lihamiso"?!
