There may be a connection to increased violence of the school shooting type, but I am not sure it is a denial of any particular religion.
First, because the immorality of such an act is known to the shooters whether a religion taught the morals or society in general taught them.
Second, since these mass-shooters know their behavior is wrong and do it anyway at such cost of innocent life, then it is not a decrease in morality but an increase in psychopathy.
Jung wrote about that very thing back in the 1950s. Yet, the connection to an increase in violence and a decrease in religious belief is still made.
Jung theorized that the post-modern tendency to deny all things spiritual left us with an incomplete sense of self, and that has resulted in an increase of new -- never observed before -- psychological ailments and personality disorders.
Of interest, I think, is that Jung makes the point that profound life-changing spiritual experiences once described angels; but, in the post modern world, describes flying saucers. Each describe heavenly beings of a nature superior to humans, but one is a spiritual marvel, and the other a technological one.
The spiritual speaks to who we are
, but the technological only speaks to what we do