Its not an invalid notion at all. It merely requires you to put yourself in what would be an unfamiliar position, and imagine how you would want to act in that situation.
You assume wrongly that we are all capable of imagining ourselves in any unfamiliar position that helps you pursue your point. This is not true.
As most of the women here will quickly tell you, most of us men are incapable of putting ourselves in the position of a woman. Not only do we lack several specific sensory nerves, hormones, etc., that greatly influence human thought and decision-making, but we also don't have half a lifetime of being raised female with all the experiences which inform our morality and behavior. This is why Mrs. Fraggle routinely says, "I'll give a flying fuck what men think about abortion the first time one of you assholes gets pregnant."
That the answer you give is never going to occur, and might even be an impossibility for you to reach in reality the position of choice offered, does not invalidate the thought experiment for you.
True. What invalidates it is that
I cannot imagine myself in that position, so I cannot perform the experiment.
This is not to say that I could not, in real life, find myself in that position. If someone killed my dog, I might become irrational and beat his brains out with a hammer, strangle him with a bungie cord, eviscerate him with a spade, or murder him with whatever other common tool happens to be nearby, before the police arrive. But it's difficult to hang onto that imaginary scenario long enough to apply a logical argument to it. As the last wisps of imaginary anger dissipate, I'm left with the sober realization that it is I who will end up in prison, leaving my other dogs to be carted off to the pound where they too will undoubtedly be killed.
The only way anyone in modern civilization can be driven to commit murder (with a few rare asterisks such as my terrorist-cell scenario) is to be so angry as to become illogical. And who gives a shit about what illogical people think?
I agree, this isn't about the death penalty, or whether there might not be something good about these people after all in their personal relationships. We are asked to imagine someone we are already convinced deserves death. I'm not a proponent of the death penalty. But I do believe in personal justice. Murder is murder, and no one should get away with it. But say if someone brutally raped or murdered one of my family members, I would not hesitate to murder them and accept the consequences. I think civilization will survive.
Yes, civilization is robust enough to survive occasional irrational behavior by its members. But will your
family survive? Your wife is already dead, and now you're in prison for at least ten years.
What will happen to your children? The shit-for-brains government will decide where to put them, and you can bet that it won't be with the relative you would have chosen. Perhaps with your ex-wife who abandoned them and ran off with a guitarist. Perhaps with your mother, who beat you back in the days when parents could get away with that. Perhaps with a court-appointed guardian, someone you haven't seen in years.
Everything we do has consequences. So it's always better to not be ruled by pure emotion, especially a negative one like hatred, anger, revenge or religious faith.