This sounds like the title to a John Grisham novel, or a Harrison Ford movie. but it is not.
The passuk tells us in 17:7 that if one worships other gods, etc., we bring him t judgement with witnesses, etc., and when the death sentence is meted out, "the hand of the witnesses will be first to kill him, and then the hands of the ntion, You shall remove the evil from your midst."
Why must the witnesses get their hands dirty and also play the role of executioner? Why not have the executioner of the Beis Din perform the execution?
It is a fairly easy thing for two people to show up in court and testify against someone else (truthfully of course. I am not talkign about them possibly fabricating their stories). There would be no reprcussions and nothing for them to worry about. By making them follow through and carry out the execution, we are making it much more serious. They have to realize that they are not one small part of a process, but everything is hinged on them and they will be the ones up front getting their hands dirty performing the execution.
If that does not sober them up, nothing will.
They, and the rest of the nation if necessary, must be prepared to act on their findings and eradicate evil from amongst them.