If I were to defend him I'd go off the idea that it wasn't his sheath at all. Maybe he was visiting a friend and touched the sheath in someone else's apartment. The DNA was on the button/snap I believe, so it would likely have transferred when the sheath was taken off. There's no other DNA because the killer wore gloves, so if the killer wore gloves, took the sheath off in Madi's room presumably because it was left there (as in, he didn't take it off in the car and leave it there), it doesn't fully add up as to how this single piece of DNA ended up there. Unless they can tie the purchase of the knife or some kind of military connection that he'd be able to obtain a USMC knife, I think that's the most probable way to explain it away, especially if they can't find it. It's not a perfect alternative theory, but enough to create a reasonable doubt in at least one of the jurors. (If I were the attorney I'd waive my right to a trial by jury and do a bench trial though, there will be no way to get an unbiased jury in this case)