[Mzyxptlk]

If you are posing the question "why, if abortion is legal, do we not force parents to have their disabled babies killed?", you're missing a vital point, namely the word "force".

If you didn't intend that word in your question, there's still a practical issue. From a moral point of view, I'd agree that abortion is about equal to killing a new born baby (as far as I know, the blank slate argument holds). But then you wouldn't be killing new born babies. Why? Because many chronic disabilities or illnesses don't get noticed until far past the point of birth. This of course means you wouldn't be killing ("terminating") a new born baby, but a young child, in which case you'd not be killing a blank slate.

As for your "wrong to kill" argument, maybe society allows the death of soldiers, but it doesn't approve of them, nor would society handpick soldiers to be shot. Society (I'm avoiding the word "we") considers soldier's deaths (indeed, deaths in general) to be a necessary evil, not a thing that should be openly advocated. If this were not the case, suicide would not be as much of a taboo as it is, nor euthanasia, or indeed, the killing of babies.

The real issue at hand here is that there is not a point at which a blank slate becomes not blank, for lack of a better expression. Society has a problem with grey zones. The development of a sperm and an egg into a person starts at conception, and ends at either death or adulthood (the transition into which is of course another such grey zone). We would love to be able to point at a specific point in everyone's life and say "see, that's where little Johnnie became a person!", but there is simply no such thing. Radical christians would place this point at conception, pro-choice groups at birth, sociopaths not at all, and they're all wrong.