The line between murder and self-defense is incredibly vague. From a legal standpoint, if you read laws, it looks more clear-cut, but in practice it almost never actually is. Even if someone seems to have a clear case for the former rather than the latter there's almost always someone willing to argue mitigating circumstances.
On your plank of Carneades question though, either person is technically a murderer, if you want to define murder as 'killing of another willfully'. Is it also self-defense? Almost definitely. The legal system would call it one or the other in order to assign a sentence, but ultimately it doesn't matter what anyone else says - it's something the survivor would carry with them for the rest of their life. The feelings of either the survivor or the deceased are ultimately inconsequential, because while the survivor might justify themselves they also know exactly what they did, and no amount of self-reassurance will change what's happened.
Humans live on the deaths of other things. Whether it's wood to build homes, animals to provide food, or other humans to provide whatever real or imagined resource that's gained from conflict, humans are creatures of self-preservation because humans are animals, and nature is an entire system of fighting to hold a position over some other form of life. Ultimately there's always some sort of life-form that's 'shoved off the plank' in order continue living.
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On your plank of Carneades question though, either person is technically a murderer, if you want to define murder as 'killing of another willfully'. Is it also self-defense? Almost definitely. The legal system would call it one or the other in order to assign a sentence, but ultimately it doesn't matter what anyone else says - it's something the survivor would carry with them for the rest of their life. The feelings of either the survivor or the deceased are ultimately inconsequential, because while the survivor might justify themselves they also know exactly what they did, and no amount of self-reassurance will change what's happened.
Humans live on the deaths of other things. Whether it's wood to build homes, animals to provide food, or other humans to provide whatever real or imagined resource that's gained from conflict, humans are creatures of self-preservation because humans are animals, and nature is an entire system of fighting to hold a position over some other form of life. Ultimately there's always some sort of life-form that's 'shoved off the plank' in order continue living.