The cadaver note
I think people are giving Durst too much credit by suggesting he cared about Susan, so he wrote the note so her body wouldn't lie there rotting.
He wrote the note purely as an alibi. He even said it. "How could I travel from x to y to x to y again". It makes it look like if he couldn't of got to LA and back again the he couldn't write the note.
But we now know better.