Why I don’t think we quite know. However, the fractures first showed up with the whacky Cylon virus: Helo wanted to save them, and Sharon was less than supportive because she wanted to follow the orders and the leaders that Helo wanted to ignore.
I’m also thinking that, up until the New Caprica rescue, their relationship existed in a little bubble where it was just the two of them against everybody else. Now, Athena’s got another life outside of Helo, and they’ve now got a kid to deal with. Not as much wifely attention for Helo, methinks.
Urgh. Just realized: it’s a mom-gets-a-job AND a kids-take-up-mom’s-time alienation of affection story. It’s like sci-fi Thirtysomething, here.
You know, I think there’s a lot too that. He held the woman he loves in his arms and put a bullet in her belly. Sure, intellectually he knew she’d be resurrected and all, but there’s got to be some part of him that’s still shrieking over the horrible, horrible crime he committed. And having her back after they spaced or cremated her body has to be setting up all sorts of conflicts in his mind. He’s got to have a lot of anger towards her for putting him in that position. I actually hope they address this further in a future episode.
I think Lightray and muldoontheif raise good points.
Athena wasn’t in favor of Helo killing the cylon prisoners nor was she in favor of Helo’s latest crusade for the Sagitarrans. She wants to fit in with her crew mates and (as much as possible) get past the fact that she’s a cylon. Helo keeps going against the group and so reminds everyone that they’re different.
Plus, killing your wife (especially in a violent, dramatic way like shooting her) isn’t something humans are used to doing (at least I hope not)! Even though he knew she’d resurrect, it’s not something I can imagine a human brain can readily deal with. I know many parents who have a hard time watching their child go through painful medical procedures. Yeah, they know it’s for the child’s good, but seeing someone you love get hurt and not stopping it just goes against your instinct. Shooting has got to be much, much worse!