I understand completely. He denies her existence as someone he is actively engaged with. He projects an image of upstanding moral Mormon. It helps his image, status and business. He is a marketeer, after all, and apparently pretty good at it. Everyone believes him. She is a pariah. Since she is a secret and isolated, who would believe her?
In my mind, that is the root cause of her rage....humiliation, despair and zero validation that she is worth more than a 2-bit *advertiser censored* in the eyes of everyone. Yet he actively continues to engage in a relationship with her. I get it why she didn't/couldn't break away, but many who are unfamiliar with emotional abuse probably won't be able to understand.
But it doesn't justify his murder! Unless it can be shown there was a pattern of violent abuse (IMO) and that she was "reasonably afraid" (vs psycho raging).