Gavra, the chilluk is that r’tzichah is talui in the metzius: was he someone who would have died within twelve months, or not? The onesh misah is only on one who kills the latter.
Treifah by an animal is a din that a treifah is assur. The definition of treifah, as per halachah l’Moshe miSinai, is a nekev which would kill an animal within twelve months [at the time Chazal codified/established the parameters for the din].