"Yeah, its me. Don't cream your pants"

One thing I know to keep in mind is that the book is told from Max’s perspective, so it's her eyes we experience the world from, and her opinions that guide us through the story. For that we have to value her as a somewhat unreliable narrator, considering she is thirteen at the time and dosnt understand certain things.

A quick timeline to keep in mind because the book handles time very messily:

Billy has just turned 17 by the time Max and Billy first meet.

After Halloween in season 2, Max says she’s known Billy for 7 months, which takes us back to early May.

They move in together three weeks after the wedding in late June, meaning Max and Billy have lived together for a total of 5 months by the time they get to Hawkins.

Neil confiscates Billy's car keys for two months in June, so I’ve taken an educated guess that Max first witnesses Neil's abuse somewhere around august, then in September/October Billy breaks Max’s friends arm, leaving for Hawkins late October.

Now onto the deep dive:

In 'Runaway Max', we learn about the terrible Hargrove family dynamic, and how the Mayfield's learn to navigate that. Max gets a very graphic front row seat of Billy's abuse in chapter 10, and during that chapter Max responds to that situation as any 13 year old would, scared and confused. Despite this however, Billy's image doesn't change in Max’s mind. She has no visible compassion for Billy at any point of the book except from this chapter, and after that she states that she’s actively trying not to care about “his stupid life and his cruel dad”. 

From a writing standpoint, choosing this to be a part of Max’s character takes away from her complex experiences in an abusive household. Yes Max is a hardened character, but not exploring these difficult topics of disliking Billy while also feeling sorry for him makes her feel like just another pawn for the audience to make them dislike Billy. The writers could have made the step-siblings dynamic much more interesting to have them navigate this terrifying experience together. But I understand the duffers just wanted another one dimensional antagonist for season 2.

"I'd watched the Hargroves in action. Neil standing over billy with the belt - calling me a stupid little girl - making it clear that he thought I was weak and pointless. Knowing Neil believed that still wasnt as bad as the way Billy had hated me for trying to help him"

I have mixed feelings about this. I feel this description Max gives dutiful to anyone going through that situation, that they would feel disheartened by someone rejecting their help, and verbally berating them for it. However, it’s vitally important to understand the context of Why Billy reacts to Max this way.

During the assault on Billy that Max witnesses, Max calls out and interrupts Neil, trying to diffuse the situation. Neil responds not to Max, but to Billy “Is this the son I raised? A worthless loser who needs a little girl to fight his battles for him?” And then strikes billy again. 

Max assumes this to be an attack on her, however that's not what's happening at all. Neil is using Max against Billy. He takes Max’s intervention as a sign of Billy's own weakness, a softness. “Any hint of softness and he would never let me forget” In a way Billy had been trying to teach Max to harden herself so that Neil couldn't find anything to target her for, Billy had been making her more likeable for Neil. And now this softness that Max is showing for Billy, by standing up for him, is getting him punished. It’s been implied before that this was the case, but now we are seeing it explicitly that Billy is being punished for Max’s actions. This chain reaction forces Billy into a position where he cannot be on Maxes side, he cannot be friends with Max, because siding with anyone other than his father equals punishment.

After Neil leaves, we circle back round to the sentence “Sometimes Billy acted like we were in on some big, crucial secret together / like we were in some sort of secret club together - like we could be on the same team” Billy had been trying to tell her from the beginning, they were both members of a club trying to navigate life with Neil Hargrove, bonded by shared experience. They were supposed to be on the same team too, victims of Neil, but Neil has made that impossible by using Max against Billy. “I could see all the ways he hated me” Neil isolates one from the other, Billy resents Max for getting him hurt, and Max resents Billy for rejecting her.

During most of the sections of the book that happen in Hawkins, Max continues to call Billy a “Monster”. However, never Neil. It’s not untrue that Billy has a mean streak, he can be cruel and heartless, most notably when he breaks the arm of one of Max’s friends. The only time this level of violence is seen in Hawkins is during the fight with Steve.

Both of these big outbursts of rage are built up by attacks from Neil. Note: Billy is still an asshole, these are not excuses for his actions only explanations.

Yes, there are two occasions in which Billy grabs hold of Max’s arm, but there is an argument to be made that this is just normal sibling behaviour. Have you never pushed or shoved your sibling before, or been on the receiving end of that physicality. It’s not always pleasant, but it’s not uncommon for siblings to get physical during disagreements. Max is also only distressed by this on the first occasion, “He caught me by the arm, and it wasn't the first time he’s ever touched me, but other times had always been to push me out of the way in the kitchen or flick me on the end of my nose. This time, his fingers closed hard around my elbow” - however the second and last time goes like this “He reached out fast and caught me by the wrist”. Those are the only times Billy is ever physical with Max.

Despite this, Billy is still the monster. “Billy was the closest thing to one that I had ever known - this was what it meant to live with the monster.” Monster singular. Billy is the most terrible thing living in the Hargrove home, not the man who beats his son.

Personally I find something off putting with Max ranking Billy as a worse monster than Neil. During the night at the byers she says "I understood now that Neil was in his head, and that meant he was just as dangerous as his father. Worse because Neil was cruel and frightening but he cared how things looked on the outside. Billy was crazy" Max isn't stupid, she knows Neil is a bad person, and maybe from Maxes point of view Billy is worse than Neil because Neil hasn't ever physically hurt her. But from a writer's point of view, to say that the victim is worse than the abuser? That is both dangerous and honestly disgusting. To call Billy crazy, and insinuate that he’s acting like a ‘bad victim’ because he doesn't pretend that everything is normal is so hurtful to victims of abuse who see themselves in Billy. 

Continuing this thought, Max then goes on to compare Billy to a recently possessed Will. Which, first off, comparing abuse is never okay. But what Max says is almost worse, "Will had turned into something terrible and frightening, but even with the mind flayer working through him, he was trying not to let it. He'd almost gotten us killed, but you couldn't blame him because he didn't ask for this. He was trying so hard to stop it" Is the author trying to say that Billy should try harder to not let the abuse he has been experiencing at the hands of his father since before he was ten, affect him? Because if Brenna Yovanoff is using Will as a ‘good’ example of a victim of parental abuse, and using him to discount Billy's own experiences, then I’m sorry but who let this book go to print?

Obviously as a character Max choosing this comparison means very little to her because she doesn't know about Lonnie, but the writers do. Comparing Billy to will is a choice.

Billy and will have both experienced physical abuse from their fathers. Will is on one side of the spectrum of victims, quiet, timid and apologetic. Billy is on the opposite end of that spectrum, his experiences have hardened him, made him angry about what's happening to him. Billy isn't quiet, he’s an asshole and he has issues with authority, but the one thing that sets Billy and Will apart is the fact that Billy is still experiencing that abuse.

Will is a survivor, Billy isn't.

To imply that "you can't blame Will because he didn't ask for it", but it’s okay to blame Billy, does that mean we are supposed to think Billy is asking for it?

There are choices writers make in the information they reveal to their readers, the phrasing that is used and the comparisons they make. It speaks volumes that while Will is praised for his experiences and bravery with his dad, Billy is called a monster for acting out because of those same experiences.