You're imposing an enlightened modern viewpoint onto a universe with explicitly different rules. In Middle-earth, there actually IS such a thing as absolute evil, unredeemable and not possessing what we would call a soul, and the orcs are plwced firmly in this class of being. Their sapience is not relevant to the morality of killing them when they are evil.

I understand that this doesn't map onto the real world very well, but the real world also doesn't contain immortal beings who are within a few degrees of separation from the creator Eru Ilúvatar himself, who have literally spoken with either him or his greatest servants the Valar. It's hard to deny the rules of good and evil when you have them firsthand from the creator of the universe.

My point is simply that you have to define the frame you're arguing within. If your frame is the real world, then you are correct and orcs should be treated the same as any other living being. But if your frame is the subcreation of J.R.R. Tolkien, you must acknowledge the stated realities of that world.