But its simply being alive isn’t enough to justify a claim of ‘murder’. There are many living things we kill readily without considering it murder. And even on that count, it is not the case that an undeveloped fetus could survive outside the womb, so it’s not entirely clear that an early fetus is a fully separate living organism.
I’m not sure what you mean by ‘characteristics’ of being human ... fetuses are remarkably unlike humans until quite late on, and in any event clearly we don’t want to say that killing something qualifies as murder if it looks like a human. A brain-dead body also looks like a human, after all.
Again, the salient fact has to do with the development of the brain. The brain has to be fully enough developed for murder to be possible. (Precisely where the cut-off is is something we could leave to scientists to decide).
I’m not totally sure what you mean by your last sentence though, so maybe you should elaborate.