Atheists, the death penalty and abortion...

[Esquilax]

If a zygote/fetus is not a complete human life, what makes up the other portion of that life?

Thank you, and that's a good question. The zygote only gets one as far as the body, the physical makeup of a person. But that's not what we mourn when a person dies; the body is just a vehicle, after all. The other portion of life is experience, a mind that thinks, senses the world around it, and develops a personality. We as people are nothing more than our personalities and memories, after all, and a fetus has none of that, until it's born and starts actively experiencing the world.

If you value something about human life, is that value limited to "completeness"?

No, because I don't think there's a point at which a person ever becomes complete. But the value is in the person as a constantly evolving being; putting special significance on the zygote is akin to putting significance in the container. It serves a purpose, for sure, bodies are important, but they aren't us.