Harsher punishments for mass murder? I don't think any mass murderers are getting out of prison on early release programs.Most of the statements already made here are accurate to your "means to an end." Gun legislation will not prevent nor deter further incidents. What will?
Much more harsh punishments is a start. Early out for good behavior and other BS reasons should be eliminated. More of an "eye for an eye" system would instill the fear necessary to deter violent crime. There will still be some exceptions, but they will be fewer for sure.
The majority of the other exceptions could be better handled by more intensive psychological research and enforcement of surrounding psychological policies. This of course should involve psychiatric professionals to determine, not lawyers, judges, and any other random person someone sees fit.
Indeed, there will still be exceptions even after instituting these two things. However, they will be MUCH less fequent.
The only way psychological policies could do anything is if everyone was subjected to a test before they were allowed to own a gun. Is that what you are advocating? You want to leave it up to Psychologists who can and cannot own a gun?