Let's say that we kill blastocyst, and that's ok. Why not killing something like that, which is merely 5 seconds older? Seems ok.
And after that, why not kill something that's 5 seconds older, etc, etc, all the way up till birth, or even after. So, there's a continuum there that's very difficult to ignore in context of development. We may choose to abstract that continuum by time-jumping, and say well it was not here, but now it is,but it doesn't magically do away with that continuum and rationalization solely based on characteristics of certain maturity state, which isn't clearly defined for that particular reason.
If we reverse the process, and say "it is now a person", then would that be the case 5 seconds ago, and 5 seconds before that, etc. On certain continuum it would be difficult to make that distinction, because a difference between each 5 second interval is minimal. Thus, at any step of the way, you couldn't clearly say as to how it would be so drastically different from 5 seconds ago that it would justify your label at this moment in time and not 5 seconds ago.
I hope it makes sense.
We can clearly make that distinction at conception. After conception, it's not something that we can do.