If you want to make some laws that might deter a future mass murder or two, try making it illegal to release the name of the murderer(s). That way they gain no fame. Make it illegal to release any of their writings or videos about why they did it, so they can't use mass murder to "get their message out." Make it illegal for anyone to produce their stories as movies, books or TV shows.
Then anyone who commits this kind of crime dies in obscurity and just goes away.
Of course that would be attacking the first amendment, and those who prefer to attack the second amendment would scream like stuck pigs.
Just wishing, I guess.