Pertaining to:
#3: I merely am saying that all points in an individual's life are of similar worth. As far as whether ANY of those moments of life are worth allowing to continue, that is a separate issue in my opinion. For a quick answer, we humans general frown upon murder because unrestricted killing would usually not be a positive for the survival of the species. Others may claim murder was made illegal due to religious reasons, but those religious reasons originated with our DNA's survival programming. One may question why we humans should afford the human race special treatment, as compared to other species. My answer to that is that humans are the only instance we know of where the cosmos has become self-aware. That alone is reason enough for us to continue as a species, for now.
As far as sentience, it is true that it does not exist early in human life. However, it will most likely develop, if the individual is allowed to continue to mature. You put fetuses together with coma patients, so I'm confused when you say "The first group enjoys no expectation of being allowed to continue." I'm assuming you mean most coma patients and a very small percentage of fetuses.
#4: I think that is a bad direction for us to go. Once the rules allow culling of human for whatever seemingly beneficial reasons there may be in the future (i.e. population control, elimination of non-productive members of society, reducing the number of people with a higher likelihood of costly health issues, getting rid of pain-in-the-ass children), our society will have become a very bad place.
#5: This was just a tongue-in-cheek comment relating to killing certain groups of people off.
#8: What I mean is, if our society makes it illegal to murder humans, then it should be illegal to murder humans. What is the rationale for discouraging murder of a human at one stage of life, and not at another?
#9: Absolutely, that's the main reason. We have laws against it. Abortion allows it in the case of the unborn.