If they both undressed for sex there would be minimal blood on his clothes. He could kill her high on drugs and then get dressed. I don't know. Post-murder I do blame the media in the interim, whoever the culprit was.
This is also where my thinking is going. One problem with it is that it means she would have to be found re-clothed, which ought to compromise him forensically, yet no blood prints were found. Maybe he 'cleaned' the scene? I admit it's starting to become overly-contrived now, like I'm trying to make the facts fit the suspect.
So far, for him to be guilty, I have him wearing two layers of clothes, with a green German Army parka-style jacket on top.
They meet at wherever it was along the path, behind the wall, and begin undressing, and just as he has got her vulnerable, he attacks her.
He kills her.
Then he re-clothes her as necessary, being careful not to leave any blood prints. He doesn't care about latent fingerprints and DNA because he's her boyfriend and, all being equal, such evidence cannot incriminate him.
He also doesn't care about transfer blood stains from his own bloodied clothing, as it's her blood anyway. There is a forensic risk here, due to possible fibre transference, but he doesn't care about that either as he has already taken his outer clothing layer off and will dispose of the compromised clothing later. (Nevertheless, there is still a risk of secondary transfer fibres from other clothing of his, including the outer layer he has already taken off, but he probably doesn't think of it, or if he does, takes his chances).
He ties her up because he needs to stage this as a non-spontaneous killing. His reasoning is that this should deflect suspicion from him (it doesn't, but that's another matter).
He then conceals his own bloodied layer of clothing with the outer layer and parka (or, in the alternative, he takes the bloodied clothing off now and conceals it somewhere there and then, to be collected later - though that seems doubtful, for several reasons).
He then returns home, but stops along the way at the gate to the path at the Newbattle end, affecting to be waiting for somebody. This may be due to a vaguely-formed (and badly-thought out) idea that he needed to be seen standing around casually, so as to minimise suspicion on himself.
If, as seems likely, he is still wearing the bloodied clothes, he takes them off at home. He then lies to his brother about what has happened, perhaps saying he acted in self-defence, and his brother then burns the clothing. Or his brother is oblivious and Luke has disposed of the clothing some other way. At this point, he makes the decision to dispose of the green jacket as well, even though he was loitering around deliberatively in it at the Newbattle gate. He also has to dispose of the knife.
There is the beginnings of a pro-guilt scenario. Probably there are holes in it, as I am still researching the case. It's probably also, as I say, overly-contrived and maybe too complicated for a 14 year old.
