I figure if someone wants to call himself a Christian, who am I to tell him he isn't? I don't have a dog in that fight. The intellectual and historical aspects of the debate are interesting to me—that's all. As nearly as I can tell, all those who want to call themselves Christians can't even agree on what to look at to determine whether one is properly Christian or not. If you can't even agree on what the criteria should be, then it doesn't matter so much whether one group says another group hits or misses them.
The problem here is that the Trump regime was originally dead-set on calling this tragedy an instance of anti-Christian violence. They want to shoehorn it into the grievance narrative that says Christians in America are being heavily persecuted, ostensibly by non-Christians. But if the perpetrator turns out to have specifically targeted Mormons from a position of a mainstream Christian objection to their specific doctrine and practice, then it's just religious infighting and not the atheist-on-religion violence the Trump regime wants to project.