I’d be a bit cautious at saying this was Christian Nationalism in a real sense; it’s likely that the person harbored bigotries towards Mormons but would be unable to articulate how shooting them and burning their building advanced a political-religious agenda.
I posted upthread but to clarify more: these people (Kirk, ICE, LDS, etc) are not Gavrilo Princip or John Wilkes Boothe or ISIS or Al-Qaeda. They do not commit horrific acts of violence with an aim to shaping external reality or influencing others.
They are angry, hopeless, and marinating in the most toxic stew of simmering resentment and violent currents that humanity has ever created. They express directional grievances and sympathies but the anger is what’s centered and the “cause” is an accessory.
Their outbursts have horrific effects on other people. But ultimately they are acts of violence designed and carried out to disrupt their own internal worlds— not the external one. It is socially contagious but the target is really the host.