fucking Dean up as much as humanly possible
Wow. If John dies now ... what potential to fuck Dean up. Because that shifts "fault" for John's death from Sam to Dean.
If Sam had just gone ahead and shot John - which he totally would have done, if Dean hadn't been lying there in a pool of his own blood, pleading for Sammy not to - then it would be on Sam. And honestly, I think Sam is capable of carrying that.
But Sam did what Dean wanted. And that let The Big Evil escape. The Big Evil that then came along and smashed them with a semi. And if that crash killls John ... then the events that kill John were set in motion by Dean.
And I'm not sure how well Dean's equipped to carry that.
Oh. Oh, fabulous.
(Jesus, I'm the only totally sadistic ass who's COMPLETELY looking forward to as much pain and angst as possible in S2, aren't I?)