you probably wouldn't see me use that line of logic though...
imho the death penalty is about society being better than those it gives itself the right to imprison, rather than the fact it could permanently end the life of someone. I grant you, it is an argument, just, ya, I try to avoid things that deliberately try to make the issue more confusing than it is
the other answer would be, the chance our legal system is wrong is actually pretty great. The chance that our lord God infused the embryo with an enduring soul that guarantees a right to life at penetration is laughably small from any empirical perspective, which I am biased toward. The only way such a window isn't an acceptable approach is if you believe in some sky-father actively putting souls in lumps of cells, otherwise the whole notion of some "moment" where life begins is nonsensical. Its like asking where on the spectrum of visible light red starts and orange stops.
additionally, "guilt" and "life" are much different concepts. someone did or did not do the action they are accused of. In a perfect system, this would be knowable. life is not binary like this, as there are many things that appear to be somewhat alive and somewhat not. Further, life is entirely an anthropic construct that we have decided ourselves, not something like guilt that describes an action (ok, we say it is wrong, but otherwise, the action occurred no matter how we describe it). To say there might be a definition of life outside of the human definition for life, or the one that humans use to define a window of when a child might be alive, really is only possible if you think there is an intelligence behind life in the first place, which I don't and I fervently think our legal and governmental system shouldn't.