Great question and I’ve thought about it myself.
If he was anti-vaccine obsessed, perhaps one or more of his family where planning on getting vaccinated or already had been and he just found out.
I can only guess based on conversations here with anti-vaccine people.
Some believe the vaccines release HIV.
So who knows what was in his mind.
True, we will never know what was going on in his mind.
A number of Q followers seem hyper-sensitive to anything they perceive as a slight or criticism of their beliefs, and lash out with surprising viciousness. Many have been cut off from family and friends who tire of their obsessions, which must be very painful. Many really have been mocked and ridiculed by those who do not share their beliefs, and I can see where that would get to them.
Who knows whether this guy went over the edge and lashed out with deadly force at his wife and daughter because he believed they had criticized his beliefs? He was probably mentally unstable to start with and might have found something else to obsess over, lizard people or contrails or climate change or whatever.
Whatever his demented “reason” for murdering his wife and daughter, it is such a terrible and tragic thing to happen to any family and I pray for them.
Sometimes there is no answer other than people let their beliefs become an obsession. Like the Bernie bot who shot up the GOP practice session.