There is a theory on what caused the massacre to take place, called the perfect storm theory. It describes how a number of different factors came together to allow it to happen. Now we can take all of your points into consideration here, and others.
- Desensitisation through games, films and various other media.
- Avaliability of firearms/weaponary
- Their motives
Let's just keep it at those three points for the sake of this discussion.
Now, as I see it, the first two points are common in most people. It's natural to become desensitised when directly or indirectly involved in violence. You will never, ever stop people from being desensitised, because you simply cannot totally remove violence from our world.
Again, the avaliability for firearms and weaponary was more or less common to everyone in their particular state.
Their motives, were something that was not common to everyone. Their motives are the base of the cause. If you remove their motives, it would not have happened.
Yes, the other factors played a part, but there's millions of people around the world which share those same factors who are considered perfectly sane and law abiding. The difference is they don't have the motives.
Removal of any of the other factors arguably, might have stopped the massacre from taking place. But removing those factors, is removing our freedom. That's like accepting the problem, just removing the tools. That's not the right way to go about it in my opinion.