Franklin Plaskett is not the man Eva expected to marry. He's an American. A meat-eater. A Republican (4.6). And he turns out to be a terrible, manipulative father—certainly not what Eva was expecting when the couple finally decided to have a baby. Before Kevin is born, the lines are drawn, and Franklin is firmly on Kevin's side.
As soon as Eva announces she is preggers, Franklin turns controlling, forbidding Eva from drinking and dancing. When Kevin is born, Franklin never believes Eva's accusations that their son is a horrible troublemaker. To her, Kevin is out to get her. To Franklin, Kevin is "crabby" or "fussy" (9.31). He's a terrible enabler of his son. In fact, Franklin's biggest strength is diminishing everything Eva says, or completely ignoring Kevin's numerous warning signs. Eva observes, "What strikes me now is not your foreboding, but your capacity to ignore it" (13.30).
In this way, Franklin creates a rift between Eva and the rest of her family, and for this reason, Eva ends up harboring resentment toward him. At one point, when Franklin decides not to punish Kevin, Eva remarks, "You sure didn't burn out that fast when you were mad at me" (21.53). It's as if Eva and Kevin are siblings, not mother and son, and Eva is bitter that Kevin gets favorable treatment. Because Franklin treats his wife like a child, he is both a terrible husband and a bad father.
It's hard to understand why Eva forgives Franklin for how terrible he is to her. With all the travel imagery in the book, Eva and Franklin are like two countries at war, except Franklin forms alliances and uses them to assault Eva with bombs and shrapnel. Yet, in the end, Eva, damaged and broken, forgives Franklin.
Why? Maybe it's because Franklin loves Kevin, which is something Eva herself does not. Or at least he's trying to love Kevin, something Eva has a hard time doing. We never get Franklin's point of view, so we don't know how much of his Leave It to Beaver-type father/son interactions are genuine and how much is just Franklin trying to get on Kevin's good side or convince himself that Kevin's actually okay.
What is clear is that Kevin hates Franklin and doesn't show it. And Franklin either doesn't notice this or completely ignores it. He doesn't seem to take issue with Kevin calling him "Mr. Plastic" (21.94) to his face. He only notices when Kevin yells at him outright, like when he shouts, "I've had it up to my eyeballs with heart-to-heart father-son talks about aspects of my life that are none of your business" (27.23).
Even then, Franklin seems to suppress the bad stuff. The most tragic moment of Franklin's life is his last. After Kevin kills his father with an arrow, Eva finds Franklin's body, and she observes, "The expression on your face—it was so disappointed" (27.170). Is Franklin disappointed because he finally realized how awful his own son is? Or is this look of disappointment an expression Franklin had all along but always tried to hide while alive, under the mask of the good father?
While Kevin's murder of Franklin is horrific and inexcusable, it is important to try to understand why Kevin did it. After all, that's the point of the book: to understand the other side. Eva's rationale is that Kevin had put up a false personality all along. She doesn't know why he did it, but he did—being a good boy to his father, and a terrible son to his mother.
Maybe it was a test. Eva, as unhappy as she is with Kevin, accepts him for whoever he is. Franklin, though, wants Kevin to be the ideal son. So Franklin is thrilled when Kevin appears to be the perfect child, even if it's an all act.
Eva observes, "Do you ever consider how disappointed he must have been when you accepted the decoy as the real thing?" (19.38). Kevin knows he can't be himself with Franklin.
When Franklin suggests divorce to Eva, it doesn't seem like anyone was expecting this, not even Kevin, who didn't realize his actions would have that consequence. So he kills Franklin instead of living with him. Eva believes that was Kevin's only choice: he could either pretend to be the perfect son forever—and without his mom around, Kevin would have no outlet for his evil side—or, "only one eventuality must have seemed worse, and that was living you with, Franklin. Getting stuck with Dad. Getting stuck with Dad the dupe" (25.256-25.257). That's a grim realization.