Only Artists Left Alive

Maybe that wasn’t supposed to be the effect that scene had, but it did. And they secured that spot in my heart when they had him immediately tell the gang leader what he wanted to know when they stopped torturing him and threatened to torture the dog he’d taken in. And this was before they even gave his whole tragic backstory part.

They didn't have to include all the parallels between Karen and Frank in DDBA s2, but chose to do so for the "Karen is turning into Frank" character arc.

They didn't need to include Karen in OLK and instead could have had that scene between Frank and Maria, yet they chose for that pivotal scene to be between Frank and Karen. They chose Karen to be the one to pull Frank out of his darkness. They chose Karen to be the one to set Frank up for what he needed to do next and who he needed to become. They chose to show that Frank has romantic feelings for Karen and that he's in love with her.

Considering Frank never moves on from Maria in the comics, I don't see why they would have him move on in the TV show to not make Kastle happen. It doesn't make any sense. Sure, Frank not moving on from Maria is a defining aspect of his character in the comics, but so is Karen being dead a defining aspect of her character, yet the writers chose not to go that route in DD s3. Frank and Karen don't even interact in the comics, yet they're frequently together in DD s2, The Punisher s1, The Punisher s2, DDBA s1, and The Punisher: OLK. They can clearly change things around as they see fit, which is what they seem to be doing with both Frank and Karen's characters.

Putting Kastle together to kill off Karen also doesn't make sense because what would it serve? To further break Frank? He's already broken from his family's murders. He doesn't need to be further broken to serve any meaningful plot points. And saying that Karen's death would break Frank is just proving that he's moved on from Maria and is in love with Karen, otherwise, her death wouldn't impact him the same as Maria's.