None of the above examples are mass murders except one. They were all individual revenge killings, that is the difference.
No matter how you try to slice it there are MANY more mass murders now than before. Many factors come to play IMO but ultra violent video games played constantly by an altered mind surely isn't a good recipe.
Time for the Mall Ninja list:
Beretta 92fs, Springer XD9, High Standard Model HB, RRA bull bbl...aw heck with it time to go plink