Does anyone really believe that, once it's got to that stage (whatever that stage may be) to kill a viable embryo is not killing a human being?

Yes, I do, more or less. To carry it to the extreme, medicine will be able (though I hope, not willing) some time in future to generate human life from a single cell. Each wanker killing a billion possible lifes daily (twice weekly, or whatever your preference).

Sorry about that too colourful language, but I think the onset of human life to be fully considered as such by the law is merely a question of definition to be agreed upon and not a good question for theorists.

Onset of pain perception seems a good definition to me, though others may disagree.

Wolfgang