I think you will agree that it is generally wrong to fire a gun in one's urban back yard.
I think you will also agree that there are very rare times when overriding reasons necessitate it.
No, it depends on the backyard, the gun, the intent of the firing, and what's on the other side of the target. the metaphor is completely invalid, this is Keyes going back on his own word, once somebody does that their word can never be trusted again. His position has proven to be negotiable, he can't ber trusted to keep his pro-life position now, he can't be trusted to keep any position, by making this "one time exception" he has given himself over to situation ethics and negotiable morality.