Here in the wide open spaces, folks listen to yodeling bands at the local bars, chew on hay, say things like "jaw-jack" and "squirrelly," affectionately refer to violent fistfights as "the Appalachian Boogie," eat calf brains with eggs, and have jack-off contests to prove their manhood. Old-time country singer Hank Snow is a household name even among high-schoolers, and simple drinks like whiskey and water have cute local names (a "ditch," so named because ditches are irrigated). The Smith brothers are from Montana, so presumably some of this stuff is drawn from personal experience.
If all that doesn't sound bad enough, consider young Roy Chutney (The Believer's Ryan Gosling), whose mostly absent father has just died walking along the railroad tracks at the wrong time, possibly as a deliberate suicide. Budget cuts then force his high school to eliminate the junior varsity football program, and Roy's not quite good enough to make it to varsity -- in a line that will ultimately be proven false, the coach tells Roy "You ain't angry enough, and I ain't got room for 'ain't angry enough.'"
So Roy's stuck on the ranch with his messed-up mom (Kelly Lynch, in a bravely unglamorous performance), spending the days chopping wood and drinking beers (the legal age has been 21 in all states since 1982, but you'd never know that here). Then one day a newspaper delivery man by the name of Gideon "Gid" Ferguson (Morse) makes him an offer to play six-man football, which, like fastpitch softball, is an unusual variant that's practically a religion among certain subcultures that the mainstream sports media seldom acknowledge. Gid longs to be a coach, and with Roy on his team, he figures he has a chance at success.
Roy develops quite an aptitude for the game, especially when he also persuades his best friend, Tracy Two Dogs (Eddie Spears), to sign on with him and back him up. But Gid's a discomfiting fellow. Early on, we learn that he has to pay extra insurance for each game his team participates in, due to some kid who allegedly died under his supervision. There are also mutterings that he's a "pre-vert" -- not merely gay, which would be bad enough in this town, but possibly a pedophile, too.
Not that Roy notices at first, as his interests are sidelined by a pretty young bartender by the name of Skyla (The Faculty's Clea DuVall), the only person around who seems definitely smart enough to do better elsewhere. But for a young guy on the cusp of manhood, Roy's getting a lot of conflicting signals, from a now-fatherless household with a messed-up mom, to being forced by Tracy's stepdad to help him steal for a living, to the near-endless provocations to fight, and not least to Gid's growing interest in him, possibly fatherly but potentially something more base. Add in the inevitable disconnect with a potential girlfriend who wants to leave town while he doesn't, and you've got a recipe for explosiveness with few, if any, safety valves. Actually, Roy's not an inherently likable fellow, and Gosling's not an inherently likable actor, either, so it says something about how the film speaks to universally confused ideas of manhood that we can relate at all.
About the title: The slaughter rule is a provision in six-man football that if any team gets ahead by 45 points, the game is called. Since the film isn't interested in being a sports movie (the games we see are montages of tackles and little else), the device as literal plot point is minor. As metaphor, it applies mostly to Gid, a man whose life was effectively "called" by the incident with the dead kid, about which we never quite learn the full details. Morse inhabits this role beautifully, showing us a man who knows he's pathetic in many ways but still tries to hold his head up and be a man regardless, even when impulse overwhelms reason. Thankfully, the Smith brothers don't try to sentimentalize him excessively as Pedro Almodóvar does his molester protagonist in Talk to Her, but neither do they make him outright evil -- he's human, and it's even possible to like him, but you need to know where the boundaries are and clearly set them.