The fact of the matter is a central tenet of most religions is a respect for human life. The argument of these shooters is the game of life is meaningless and to rigged to be worth playing so they flip the board. Religion as stated by Marx a great critic of religion is the opium of the people keeping the citizens in line.
Semi-automatic weapons in their modern form have existed for decades (the AR-15 was invented in the 1950s for example). The problem is not the weapon, or the gun control law (which were more lax in the past) but the lack of moral values and respect for life. Which causes school shooting
There is no 'death of God' argument to be made here. School shooters are driven by irrational motivations bereft of any moral examination or cognitive introspection. They are willing to deprive others of their lives without conscience or remorse. By stark contrast, Nietzsche suffered a mental breakdown upon seeing a horse beaten to death.
It certainly be argued that there is a link between the onset of nihilism (the death of god) and school shootings. A lack of values could lead the potential shooter to question: why not? However, I highly doubt this is ever a conscious consideration, or even an influence in most cases. A more practical discussion would be trying to understand the psychology of the potential shooter and how society has influenced it.
Ultimately, disturbed individuals will find an excuse for heinous crimes if they set their mind to it. This excuse could be nihilism, just as much as it could be an extremist interpretation of a religious text. Again, the more fruitful conversation to be had here is about mental health, not modern philosophical leanings.
We get it, religious fanatics want something to blame for all these terrible events. "You didn't pray to God so your child had to die," or "You did not have faith so your son who served in Afghanistan (or whatever, could pretty much substitute this with anything) came back in a coffin." The same applies for school shootings. No, they don't happen because faith is declining. Correlation does not imply causation. What really causes school shootings is the poor mental treatment in this society, that leave the mentally unstable and scarred to do these kinds of acts.
I have never heard any school shooter use Nietzsche's philosophy (much less that specific phrases) as a justification for murder. And even if they did, it wouldn't hold water logically. The only factor that correlates strongly with school shootings is the child's access to guns. Not philosophy. Not Nietzsche. Not even mental illness.
Why would there be a connection between whoever's death of god and school shootings. I am not a christian or anything of the sort and know countless people who are the same and we do not go around shooting up schools. The people who shoot up schools are mentally sick or desperate in some way. They most certainly without a doubt do not do shoot up schools because of Nietzche's death of god. I'm sure it wasn't even on their mind. Nor was it an unconsious thing.