What you're missing is that that isn't impossible.
The actual premise is impossible, because nobody ever wants to do anything "for no reason" -- there is always some reason. Which is why you need to have the discussion, instead of assuming there is no reason. To figure out what their reason is.
Because it could be a good reason. Maybe you're stranded on a mountain with 30 people but only enough provisions for 20 and if you try to share then everyone will die instead of a third of everyone. And then killing 10 people isn't actually beyond the pale, and even if you decide against it you still need to have the discussion because you need to do something.
Or maybe it's because your friend just really hates short people and wants to kill them, which is a stupid reason that isn't going to convince you, but at least now you know what it is and can dismiss it out of knowledge rather than ignorance.
There are certainly situations where it might make sense to kill a small number of people for some greater good. The "Trolley problem," and all that.
The situation I meant to describe in my post is one where your friend has no remotely defensible or rational reason for their desire to murder ten people. Surely we can imagine many such scenarios.
I can list 10 people that the world would be better without — the likes of Kim Jong-un and Ayman al-Zawahiri. That's not too different from wishing for their death. Plausibly we can discuss whether it's a good idea for them to be assassinated, and whether the power vacuum would just cause another worse despot to replace them.
So it seems quite conceivable to me that a discussion could be had about the reasons, feasibility, ethics or geopolitical implications, or about why they believe that someone does or does not deserve killing.
I meant to describe a situation decidedly not of that nature.
Imagine our friend wants to kill ten people for no remotely defensible reason. Perhaps our hypothetically murderous friend is high on hypothetical PCP and is clearly suffering some kind of psychotic breakdown.
>interesting discussion on ethics and moral philosophy
>as long as everyone is discussing in good faith
The parent didn't mean "as a hypothetical", e.g. in a "would you rather" way. They are referring to somebody who is actually a proponent of the killing, in which case it is impossible, or at least moot, for that person to be arguing "in good faith". This seems obvious to me, so maybe I'm missing something else you're trying to say.