[imported_luckyme]

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What about the sperm that is just about to penetrate the egg to fertilize it? Naturally, it's about to become a zygote, and eventually, a person. Can I kill the sperm at that point?

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The sperm about to enter the egg will result almost surely in a person one day (but never a specific one), a little less surely than the zygote a day later, which is little less sure than 4 week fetus etc. That's why this slippery slope argument is so illogical.
By the 'logic' of the Ragin Atheist you quoted, it's hard to see why this wouldn't be murder if you kill the sperm and the egg ( makes it easy for him to see).

With every other entity, including the difference between people and human bodies in a graveyard we describe it's attributes, if a martian shows up he can read the notes and identify the entity. egg-chicken, acorn-oaktree, chair, eyeball. Sturgeon-cavier.
For various psychological reasons, some people need to set logic aside and want to equate things that aren't equal.
Boil a fertized egg and they'll have PETA after you. Roast an acorn and they kick in the "can't destroy an oaktree bylaw".

specific person The concept that this zygote will be a 'specific person' is nutso. So many random things are going to happen to it as it grows that it's impossible to tell which 'person' it will be. How flawed this DNA argument is can easily be seen with 'identical twins'. They shared a lot of experiences in the womb, almost identical, but even with identical DNA and shared womb they'll still have major differences, they are two people, not one, and not just legally.
There is no way that any set DNA turns out any specific person. Why? A person 'evolves' it isn't rubberstamped by it's DNA. Cloning Me won't create another Me, they'll be tons of differences, some very major. The same things that differentiate me from you will differntiate me from him. The 'specific person' argument is not different from "a person' argument, it just sounds better if you're explaining it to a zygote.

The Raging Ath may be just as non-existant today if his mom had an extra burrito, or a snort of good rye, never mind the flu. All the other people that 'would have been' if the million of events didn't occur after that conception have the same right to claim being murdered as the Raging.
They would exist today but for event X.

The real shame is there is a very necessary discussion that needs to take place, but it has to be based on reality and not contrived, irrational claims.