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.
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.