People kill when they shouldn't — fact. Obviously, when they do, they haven't been "deterred". If they had been "deterred", we would never know about it. For this reason, I consider talk about "deterrence" to be a smokescreen.
Skip that and let's hear the real issue.
I have three objections to that. First, people do everything when they shouldn't. This doesn't mean that there should be no consequences. Second, you do know in aggregate if deterring methods worked, because you can see the incidence rate drop. Third, the cost of committing an act is based partially on the potential consequences of the action and how likely those consequences are to occur. If a "deterrent" is removed, then the cost of the act is reduced.