I think he killed his youngest because she was his favorite. It's been a few weeks since I seen the last episode, but didn't he say something like she didn't want her to look at him like the rest of his family did? I guess she didn't want him to eventually hate him like he thought the others did, so he killed her.
& I don't buy the romance either. I think Holder had a crush on Linden, but I just can't imagine Linden with anyone.