I recognize, as others have mentioned, that consequences are generally a deterrent. (We are discussing an objectively evil act, mass murder.) I think most people are deterred principally by innate character to not engage in a mass killing spree. I don't think victimless crimes can be compared with crimes that include innate evil. I don't see how marijuana should even be illegal and think many people agree with me.
When popular support for a law is lacking it is often ignored. For example, I now work in Northeast Philly. I take Academy Road North from the I95 exit past the airport. The speed limit is 25 and I see cops follow people doing almost 50 and do nothing. 
Of course I've also seen someone doing 30 while weaving like a rug maker get stopped.
The point here is most folks will avoid evil and stick to neutral or good behavior. Criminals OTOH have evil intent. They are deterred from shooting the clerk and walking out of the 7-eleven with a handful of cash but not in the way you might think. They are deterred from doing while being identifiable. They act at night, wear a mask, exit and leave the area with haste...
In reality they are deterred from actions that may get them caught.
If you have evil intent, a plan, skills, information and resources, you will likely engage in crime. You may or may not be caught. I'm sure there is a fringe that will take advantage of criminal opportunities but I suggest 2 things:
First, that people are innately good.
Second, that the criminals are deterred more by the possibility of a victim turning on them than by the law.
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