I think it mostly plays into people's fears about it happening to them. It's difficult to wrap your head around the idea that a seemingly normal person with no discernible signs could one day just take out their whole family. You look at SW who's the epitome of a Pinterest-perfect mother and wife and it's scary. (Heck, I've been side-eyeing my husband for weeks now.

) Some people can't wrap their heads around the idea of a FA so they need a "reason"-something they can understand. Finding a way of blaming SW (even if it's not an outright blame) is a way of assuring themselves that it won't happen to THEM.
With a FA situation, though, it's never about the victim. If the annihilator gets it in their head that they no longer need their family anymore,and therefore want them gone, then that's pretty much it. She could have come home and argued with him, she could have come home and made him his favorite snack, or she could have come home and gone straight to bed. In the end, the outcome would've been the same if he'd already decided ahead of time.