Maybe the perps had plans to take both of them via boat but because Mr. D. was bleeding and would leave a trail, they left him there. Maybe SD was already loaded up when they decided that transporting both was impossible. So, they left one (for no other reason that he was not transportable) and took the other and put her in a place where they thought she wouldn't be found. Maybe we're giving the perps too much credit for appearing to be well organized. Transporting Mr. D. ended up being a problem they hadn't planned for.