I know dar. I've read some comments from people saying they're not interested in Herdin's past or in the reasons behind his behavior. I honestly think that's a shame, because the novel clearly makes an effort to show what turned Herdin into the cold man he is. It tries to explain why he makes the mistakes he does —not to excuse them, but to put them into context.
Personally, I think ignoring his past means missing a significant part of his character. Understanding where his flaws come from doesn't require condoning his actions; it simply allows you to see him as a more complete and human character.
I also find the lack of interest in his backstory somewhat lacking in empathy. Refusing to engage with that aspect of the story can make the character feel flatter than the author intended.

We always have to consider the context in which the ML grew up. His childhood was deeply traumatic (he killed his father, after that he killed her mother in front of Herdin's eyes). Nobody would go near him because he was cursed — Delmark's curse.
Nobody except his aunt, who was eventually murdered, hypothetically from Blair's mother. A dead that left him completely alone, carrying the entire burden of his house on his own shoulders at just thirteen years old.
He's then forced to marry the daughter of the woman he despises most—his uncle's mistress. That daughter was present when his aunt died and never told anyone what had happened. For years, both he and his entire house believed that Blair was protecting her mother through her silence. Ten years of that —almost half his life. He simply doesn't know how to express his emotions.
He's deeply traumatized, has severe trust issues, and struggles to socialize or rely on other people. So no, he obviously can't be considered a green flag.
That said, he loves her. Deeply. Despite her family, and despite everyone around him telling him to leave her and marry someone else (because, in truth, neither his vassals nor anyone in House Delmark wants her there), he never gives in.
He defends her even when she runs away while pregnant. Even when rumors spread that she had been cheating on him with other men.
No matter what people say about her, he refuses to turn against her. He keeps protecting her reputation and standing by her side, even when everyone around him thinks she's betrayed him and gives him every reason to abandon her. That's one of the reasons I wouldn't call him trash. His loyalty to her is unwavering.
The problem is that his love isn't always healthy. It's often driven by trauma, fear of loss, and obsession, which can make him possessive and emotionally overwhelming.
So while he's unquestionably devoted to her, that devotion doesn't automatically make him a green flag.