One reason I like that Coyle may or may not have had an incestuous relationship with his mom is that it reinforces the overarching theme of his arc, that the violence which American culture would like you to believe is disruptive and deviant is actually completely acceptable and encouraged. Coyle being in the KKK helped his police career instead of harming it, he used his abusive marriages to extort money from not just his wives but their entire families; everybody loved him in Oklahoma, even though his crimes were barely hidden. Being monstrous was apart of his job description, it made life easier for him and earned him all the praise in the world. Likewise, Coyle’s mother did what she was supposed to do in the 1920s South, she married and produced children, and then she used that position for sexual and emotional catharsis knowing that - like her son - she could get away with it.
When Coyle sees Phyllis and starts talking about how “letting a woman control your life” leads to “perversion and despair, chaos,” he sounds genuinely disturbed. His voice gets airy, like he’s dissociating, like this isn’t a cautionary tale but a memory of his. He outright compares Phyllis to his mother, in between bouts of sexualizing her, likening Phyllis to a thundercloud, the thing he associates with sexual power. That petulant sniff he does when he’s calling her his “sweet mother” is the kicker; he knows he’s supposed to love this woman, he’s proud of her, he draws confidence from her, (“mama coyle raised no nancies,”) but he’s still upset. Leland looks at Gooseberry - who was a television star working with children while secretly being a serial killer, who operated in public but kept her worst crimes behind closed doors, who used her influence to get children to abuse drugs and commit parricide - and says that that’s what his mother was like. Phyllis takes him back to a part of his life that is equal parts pain, fear and arousal, she triggers something that sits at the root of his entire conception of women. They had Coyle call her a hysterical bitch three whole times for a reason, she gets under his skin.
It is extremely common for media depictions of mother-son incest to portray the son as the instigator, a reflection of the cultural impulse towards victim blaming. But Coyle casually shakes off the residual stigma of what happened to him by reminding us that being overly intimate with your mom is normal. Lots of people kiss their mother on the mouth. He’s not willing to criticize the patriarchal family structure his mother operated within, he’s staked too much on it, so instead, he zeroes in on the way she specifically misbehaved inside of it, chalks his own suffering up to an accidental misapplication of power into the wrong hands. You gotta wonder if Leland blames his father, on some level, for not controlling his mother properly, for not dominating the household like he was supposed to. That’s probably why he considers his violent attentions to be a favor he’s doing his victims; he associates punishment inseparably with love.
Coyle, in his cut lines, describes what his mother did to him and caps it off by saying “bitch never was right in the head.” He’s performing the same cope that you see people perform every day, when they call their parents (who are legally allowed to sexually & physically assault them, and publicly chortle about doing so) “narcissists,” “psychopaths,” “lunatics,” or any other otherizing label designed to frame the abuse as an outlier rather than part of the cultural norm. Ultimately, Coyle can’t deal with the fact that the social order he was raised under allowed for & incentivized his own mistreatment. He benefits from that social order, remember, certainly more than his mother did, he’s built his entire life around exploiting it. So instead, he pretends she was a deviant. He acts like what he went through was a bug and not a feature of the system he grew up in, something he of all people should know is a lie. Women who self-select into a position where they have unlimited unaccountable access to children often turn out to be predators? Wow, who would’ve thunk! Just like there’s nothing abnormal about a cop being a klansman, there’s nothing abnormal about a mother being a pederast. It is accepted; it is allowed. Sorry.