
I get where you're coming from but I dont think this is one of those... if your ex cheats on you, obviously they're gonna be the first to stop loving you and you are the last to love them, but eventually that love will fade, ya know. And as we saw, that love has already faded.
It would be more alarming if Ash already stopped loving the ex before the ex cheated because then....why were they even dating to begin with? Was Ash just playing too? You see how it can create other problems regarding Ash's own personality if you expect Ash to be the one to not be in love first.

Mistake ?? Dude cheated because he wanted to AND ran away. Cheating can never be a mistake.
And i found it even more reassuring that he hates him for that and looks extremely angry (as he should) cause you know your man doesn't put with this shit and will definitely not cheat on him.
And in another way thanks to the ex cheating our baby boys are now together.

If im understanding what you're saying is that ash not loving hazel anymore because of something he did, not because he fell out of love with him as a person, wouldnt make you feel good, correct? Because it's not hazel as a person, but an action.
But i would digress and say that they're one in the same. Your choices as a person create who you are and your character. Your behaviors and actions are what make you who you are.
So while ash did fall in love with hazel because of his character and actions, it's also what made him fall out of love because hazel decided to do something that is fundamentally frowned upon. And then to top it off, he used ash like an Air BNB whenever he needed to.
It would be more alarming if ash still loved hazel despite the betrayal and tried go separate this action as a one-time mistake than completely cut him off.
Because then it's like tryna separate the art from the artist, but some things are just unforgiveable.

Looks to me you wanted Ash to dump Hazel, not the other way around? And it kind of upsets you that Ash was not Lyle's first serious relationship. And how Ash could have remained in the serious relationship, if not for the cheating.
But that's just the thing. You view the cheating as a "mistake" while Ash views it as a relationship ending behavior.
I think you're projecting a little how you expect Ash to handle the situation? How Ash introduced Hazel was how their relationship ended and it says nothing about Ash's current feelings regarding Hazel anymore. It was about their past. It's not really an issue to tell your new partner your ex cheated, that's why you moved on.
errr... I don't know if I'd feel good hearing that the person I'm dating mainly hates their ex because they cheated on them... I mean so it's just because they made a mistake? not because you actually stopped loving them? I don't know...