He even mentioned it in his manifesto.

No, it’s not the bloody, pistol-packing life-if-cheap and every killing comes with a quip movie’s fault that he went off on a killing spree. But the amorality and cavalier way life is treated in such pictures does tend to draw the unstable, in addition to all the folks who never shoot anybody.  Remember the Va. Tech shooter being a fan of “Old Boy”?