I don't think he cared about the whole power trip, or having the four of them together. He's not the sharpest tool in the shed. I think after he killed S he just wanted to get her out of the house as quickly as possible. I do wonder why though, once he was at the site, why did he kill Cece right in the back seat next to Bella? You would think he might have taken her out of the truck, more room, more convenient for him, wouldn't have any mess in the truck ( sucked just typing that!).
but poor Bella... Why did he make her watch her beloved baby sister die? It makes no sense, ( not that any of this does!), unless he planned to just kill them both in rapid succession, ( is the only thing I could come up with as a possibility), but for some reason he couldn't do both?