I'm going to slightly revise my belief about the "Worst Day". Not by much, but I will provide more details I'm feeling more confident about now.
Nick was so busy with his trial streaming and was neglecting Kayla and the kids more than he even usually does. She already has a low starting point for self-esteem and found comfort in another man who just showed her any attention at this point. (I am going to say there wasn't a physical component to the cheating, and just emotional. But what I will also say is that emotional cheating is far more damaging to a Narcissist than physical would be anyway.) This developed through the course of Nick's trial streaming and eventually came to a head when he caught it.
This shook them both to the core, but in different ways due to different personalities. So they both went to extremes scrambling to "fix things" in their own way. Kayla responded in the way you'd expect a low self-esteem abuse victim to respond and went into hyper-appeasement mode. She started taking lewd pictures, started being hyper-sexual, started behaving in the ways she thought Nick would want her to. Possibly emulating his porn stars.
A quick aside: Nick always claims that his sex life is great and he and his wife are just so compatible and do it all the time guys for real. But he also talks about porn and has a significant knowledge of porn. If you were having sex with your wife all the time how would you possibly have time or attention for porn? I'm calling bullshit.
Anyway, Nick responded in a way you'd expect a narcissistic abuser to respond. His ego and worldview was shattered. He went full on "Fuck you dad" in every single capacity because he finally had to face a real adversity for the first time in his life. Fuck God because God let a genuinely bad thing happen to him for once. Fuck Kayla for doing it. Fuck his kids because if he didn't have his shitty kids he could leave her and find someone else. Fuck his parents for some reason. Fuck everyone!
Unfortunately for Kayla her appeasement strategy just makes her the softest target possible for Narcissistic abuse and this led Nick down the path he is now. Kayla used to have some sway over his behavior and prevented him from going down certain paths, or at least provided roadblocks. But now she is all on board for anything he wants just please don't stop loving me. Nick has used this to go full-on abusive of alcohol, neglectful of her, throwing other women in her face and publicly humiliating her on purpose just to make sure he can keep her in this state. Because this is what he wants. He finally got the completely submissive, docile, obedient whore he wished his wife was all along and he knows there's a ticking clock on that before she feels like she's repaid her debt. But if he can just keep kicking her down he can extend that timer. Hence the pictures, hence Mandy, etc. He's going to run her into the ground until she is broken permanently or until there's an explosion and she leaves with the kids.
This newfound freedom of behavior compounds as you would expect and as we have all witnessed. Naturally it goes beyond his original desires because this kind of freedom begets new and bigger ideas of what he can do. Every time he pushes the boundary he gets a dopamine hit that wears off, but from his new position he sees a further horizon he can push the boundary toward. It repeats over and over and that's how you get from "talking slightly more dirty in chat" to "outright defending pedophiles and pedophilic movies".
I think the evidence for this is apparent and I really can't see another explanation, but I'm open to anyone else's theories. This one just makes the most sense to me based on my experience with people in general.
