So if Asuma hurts Hidan, he hurts himself. If Hidan hurts himself, Asuma gets hurt. Nothing has shown that if Asuma hurts himself, Hidan gets hurt. Asuma chopping his head off seems rather anti-climactic and uncreative as a way to sacrifice himself. I do like Nests theory that Hidan has to stay in the circle to keep the bond for the jutsu going. Pretty much Hidan gets his ass knocked out of the circle to break the ceremony bond or whatever. But whatever Shikamaru thought of, he's forming a handseal at the end so his jutsu should be the one to break the bond. Of course a twist will occur (Kazaku stepping in or whatever), then Asuma will have to sacrifice himself.