I can't say that I totally follow your reasoning but I'll try to follow through with you. You wrote "If a bechor chamor is issur bi-ha-nu-uh b4 redemption. so for sure if i don't want to redeem it, after i break it's neck it could be permitted".

When learning a Kal va'Chomer one transfers an existing Din from the Kal to the Chamur. If Bechor Kodem Pediyah is Asur b'Hana'ah how can we learn from there that after Arifah it should be Mutar ?

As for your next point "Or it can't be used because u r not willing to do the mitzvah?" Arifah is also a Mitzvah, although not as important a Mitzvah as Pediyah (Mishnah, Bechoros 13a). One has not transgressed a Mitzvah if he does Arifah. L'Halachah after Arifah it is Asur b"hana'ah since it was not redeemed, and this is true even if it dies.

Dov Zupnik

Please make clear which point of the Gemara you are referring to. I suspect you are referring to the argument among the Tana'im whether Rebbi Shimon, who permits Hana'ah from a Peter Chamor before Arifah, agrees that it is Asur b'Hana'ah after Arifah. Most Amora'im rule that it is Asur after Arifah according to Rebbi Shimon, but Rebbi Elazar says that it is permitted after Arifah according to Rebbi Shimon.

The reason it should be Mutar after Arifah might be because there is no reason for an Isur to take effect after Arifah if it was permitted before, as you write. One reason to prohibit it after Arifah is mentioned in the Gemara you refer to (Bechoros 10b); he cost the Kohen money, so the Torah takes away his money (by requiring Arifah and then prohibiting it b'Hana'ah).

-Mordecai Kornfeld