the evil part of me wants desperately to believe that sam died in ahbl because of dean—because he was too focused on getting to dean (to safety) to notice jake coming in with the knife. dean distracted him, and he was killed. if dean hadn't been there, sam would have been more alert; he probably would have heard jake coming.
and i want to believe this because it's dean's fate to kill sam, and they can't escape their fates (dean proves this when he sells his soul in the next episode). jake represents john (his military background, super-strength power, calming and paternal presence that sam finds comfort in, and loyalty to his family above all else all support this idea), and john represents duty (to dean), and duty is dean's fate. so dean's fate, allegorically speaking, is what kills sam, and dean provides the distraction that allows fate to come in and finish the deed that dean himself won't commit to.
it's like a cosmic joke, right? dean clinging to sam and desperately trying to fight against destiny only for destiny to saunter up behind them and literally stab them in the back anyway. how ironic.