Again, that is why I do not blame mental illnes. Here you need a valid reason to get a gun, could he have provided one? Here they take a psycholigical test when you want to purchase a gun, did they take one from him?
Of course it is impossible to say without a doubt that this particular case could have been prevented, but the cause of why this has happened for the 15th times, since Obama became President in the US, should be taken care of, so that this will at least happen less often.
We had one mass shooting in The Netherlands,
https://en.wikipedia.org/wiki/Alphen..._mall_shooting on April 9th 2011 and that one was definitely preventable! In the past a gun permit was denied, but they overlooked his psychiatric history, when they did give him a permit. His parents immediately protested in 2008 and alerted the authorities, that their son shouldn't be allowed to own guns, but the authorities didn't reacted as they should. So since the shooting here was the direct result of people not doing their jobs, I do believe that some mass shootings are preventable and at least some of the 15 that Obama had to deal with, were preventable.
But that is all that I can say about it, I am not an expert in these matters.
EDIT: To avoid any confusion, Tristan wasn't an angry misogynist, Anders Breivik, the Norwegian shooter, was a racist, not a misogynist. So allthough we do have mass shooters here, they seem to have another motive than most of the shooters in the US and that is where I think the culture plays a role. Anders Breivik is different anyway: he is still alive! He didn't wanted to commit suidice, he wanted to change the world! Anders Breivik is the one shooter that could have been assesed by psychiatrists and he wasn't mentally ill.