Lyman Ward is a grump. If he were around today, he'd be writing long blog rants about how feminists and SJWs are ruining the world. Yeah, he's not exactly the life of the party.
Jokes aside, we still like Lyman a whole lot, even if we do disagree with him about pretty much everything. If you can manage to look past your surface-level preconceptions of this cantankerous ex-historian, you just might realize that you've been underestimating him the whole time.
When we first meet Lyman, he's at the end of his rope. He's been suffering from a brutal bone disease that's forced him to get one of his legs amputated, and all of this has had far-reaching effects on his life. It's forced him to quit his job. It's bound him to a wheelchair. It's even ended his marriage: his ex-wife dumped him for his surgeon. That's gotta hurt.
And, in case you were wondering, yes: when a guy gets his leg amputated in a novel, it's symbolic. What Stegner is showing us is that in some way, Lyman is really hurting; he's not complete. We'll find out soon enough that the real problem isn't his leg—it's the fact that he's emotionally cut off from those around him.
Anyway, with all of this in mind, Lyman's cranky attitude starts making a whole lot more sense.
Not coincidentally, Lyman chooses this moment to start working on a partially fictionalized account of his grandparents' lives. To do so, he moves back to the Zodiac Cottage, where his grandparents lived in his youth. Lyman actually had a particularly close relationship with his grams and gramps: Lyman "grew up [his] grandparents' child" because his mother "died when [he] was two" and his father was a "silent and difficult man" (5.1.1). This move is clearly a deeply symbolic act for Lyman, but for most of the novel, he refuses to even consider his motivations.
Since we're definitely not Lyman (we swear), we'd be more than happy to discuss those motivations with you. Basically, Lyman is trying to figure out "how Grandmother managed to get through a marriage that lasted more than sixty years" while his ended in a fiery train wreck (4.6.5). Actually, another way of stating this is to say that he's trying to figure out why Susan is better than Ellen. After all, Susan would never do something as shady as betray her spouse for another man. Er, right, guys?
Lyman gets more than he bargained for when he learns the truth about his grandparents' relationship. Susan was no innocent little Girl Scout: she was embroiled in a multi-year affair that culminated in the death of her youngest daughter, Agnes. That's straight out of The Jerry Springer Show. To make things worse, Oliver completely shuts himself off from his wife from that day forward, basically "abandoning her in her grief and guilt" (8.7.85). So, yeah—their marriage might have survived, but that thing was on life support for decades.
This new information forces Lyman to confront the truth about his own life. Sure, he could keep doing what he's doing, avoiding Ellen like the plague, but maybe he could take a different course—maybe he could even, gasp, call her on the phone? Though we joke about it, this an impressive step forward for Lyman. In fact, it might even be a sign that he's "man enough to be a bigger man than [his] grandfather" by making the first move toward forgiveness (9.1.240).