From letting himself into a flat with four adults ( he couldn’t have been sure they’d all be asleep or one might hear him or use the loo) taking a sleeping girl from her bed , risking her crying out , dropping his knife opposite her home. Carrying her along a very public road, even if it was 2am, someone could have seen him and cctv certainly picked him up, leaving his dna over her body and no attempt to conceal the body. His boasting on snap chat , all this points to someone who didn’t really care about being caught or maybe he’s a very sick individual with a 16 year olds logic.
As for leaving his DNA, he may have thought his wasn’t on file so he’d slip through the net, when the police came knocking he was compliant because he had no choice. She didn’t go into the sea because the crime site was not close to the shore, he’d have had to carry her body there, was probably easier to leave her where she was.
Such a tragedy one of those adults didn’t wake or someone was walking the shoreline that morning. Poor little girl.