I'm just drawing an inference from his personality and behaviour shown to date.
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I've finally worked out. The person who needs to be grounded is Ross. It always was. This was never about Fundamentalism, that was Willis's well-plotted red herring. This is about mental developmental delay and the dangers that can bring to bystanders if a person does not receive the proper care and help.
The clue was in Joyce's shock, not that Ross was threatening them with a gun but that he had a gun at all! However, the first clue was Ross's speech patterns. He sounded a bit odd to me and now I know why. His communications skills reflect his generally low intelligence.
Back home in La Porte, all the men of the congregation know that Ross MacIntyre is a strong, reliable, loving (in a stern way) but ultimately dull-witted and slow man. He's not the sort of man with whom you could trust a lethal weapon and, for that reason, they regularly check to make sure he hasn't bought a gun and the kids all know to tell their parents if they see him with one.
Fortunately, Becky's mother was there; she was always the brains of the operation, really. Who knows what was the basic relationship? The point is that she loved him in a certain way and ensured that he was never left confused and frightened by a world with just a bit too much nuance for him to understand. Because, when a man as strong and slow as Ross gets frightened, they become violent and that could have bad consequences.
Then Becky's mother died but the local community wasn't too worried. The Browns lived nearby and were there to keep Ross grounded; to reassure him when he got confused and stop him from getting frightened and violent. Besides, Becky was going to college soon and, once she'd moved out, more permanent arrangements could be made.
That's when disaster struck. Becky was discovered in a lip-lock with her room-mate. Instead of doing what anyone in La Porte would have known to do - to call Hank Brown and let him explain things to Ross in a way that kept him calm - the Administrators of Anderson did what they were legally required to do and called Becky's parental guardian: Ross.
Ross arrives, confused and upset. As always happened when he was confused and upset, he got aggressive. He started shouting at Becky and manhandling her. He started making promises to 'fix' her, based on a half-remembered conversation with a guy in a bar who claimed that he'd heard homosexuality could be 'cured' by some doctors.
Becky ran but it is quite likely Ross didn't understand the gravity of the situation. To him, it was like the day she smashed that window with her softball and ran away to hide under Joyce's bed. He'd just need to go and fetch her... It might take some effort but that's what is asked of a Christian father. Along the way, he stopped at a gun fair. He knew that Becky was in danger and, like a good Christian father, he wanted to be ready to defend her. To die for her, if necessary.
This is the genius of Willis choosing a Ruger 1v rifle. It looks a lot like a Winchester Plainsman lever action, the sort of weapon someone like Ross might associate with the stereotype of the heroic male defending his family (thanks to Western films and TV shows). However, it is a lot simpler. Basically, Ross was too dull to get a more combat-able weapon; he just got the one that was easiest for him to operate. It's the sort of weapon someone might select who really didn't know guns but just wanted one that fit into a stereotype of 'a good guy's gun' based on it looking like one.
What about the fight with Dina? Well, how would a five-year old boy react if a smaller girl jumped on his back and started scratching him? That's basically what we're dealing with here.
So we come to now. Ross is frightened and confused. All he knows is that he has to protect Becky, no matter the cost. He isn't able to reason effectively enough to realise that he is the threat. So, what Becky has to do is talk him down. Tell him that he doesn't have to die for her; things aren't that bad yet. What she wants from him, what she needs from him is to live for her so she isn't all alone in the world.
She needs him to put down the gun. Give it to Joyce so she can stand guard until the police arrive and take him to where Hank can come and collect him and take him home.
The future? Ross did wrong but, fortunately, it is unlikely that anyone was seriously hurt. A decent lawyer might get him remanded into the Browns' custody. They'll have to sell the MacIntyre house and Ross will probably have to move into their spare room. However, that's a good thing. Living with the Browns means that someone will always be there to keep an eye on him and keep him calm; stop him getting confused and frightened in this way ever again. I can even imagine that he'll be happier not having to live alone but be in a family again and occasionally reading letters from Becky (although they'll likely have to be censored by Joyce or Dorothy to stop her from accidentally triggering him again).
With a little bit of luck and time, Becky may find a woman to marry and a confused but happy Ross (because his little girl is happy and that's what really matters) may give her away at her wedding.
He'll always need someone to ground him but it is unfair to expect Becky to have to do that forever. It's a good thing that she's got friends and an extended surrogate family to help her do so.
As a postscript, I want to confront the charge of "ableism".
I'm not saying that Ross is doing this because he suffers from some kind of developmental delay. What I'm saying is that the combination of that with the intolerant brand of fundamentalism and an aggressive culture that insists that "a man's gotta do what a man's gotta do" along with his own immense natural strength all taken together have led to this outcome.
People rightly condemn simplistic narratives for violence like 'inspired by rock music' or 'influenced by violent video games'. The truth is always that each individual has a complex narrative of influences, mental health, triggers and environment that lead them to an outcome. I've just got the feeling that this is the specific set of circumstances that has led Ross to this place and time.