Chad Daybell is a writer, and he's had nearly three years now to use those skills to come up with plausible explanations for his actions. He's had three years to take the known facts and figure out what story he's going to tell the jury to make himself out to be the victim. And I think he's realized that the only way to do so is to pin it on Lori and paint himself as just some poor, hardworking family man and her as an obsessive superfan who couldn't tell fiction from reality -- which is also how I think he's going to try to explain away the "dark" and "light" ratings conversations -- as fiction, part of his books, that she took too literally and he had no idea they weren't "cosplaying" anymore until it was too late.
I think that initially, he might have wanted to come up with some story that would absolve them both so he could have his cake and eat it, too, but being a writer, I think he can see more clearly what an audience is going to find believable and how this story is going to end if he sticks with her.
(For reference, the joint charges were Tylee & JJ's murder charges and "conspiracy to commit murder and grand theft deception" in the deaths of Tylee, JJ, and Tammy. He's being charged individually with Tammy's murder and insurance fraud on her policy.)