I feel I'm too wordy to begin with; it's always a challenge deciding what to leave out for the sake of (some semblance of) brevity.

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.