I think there are two details about this "offer" that could be either clarified or added, and might change the minds of those who say "Hell yeah I'd do it!"
One: it should not be a man, but a person. So theoretically you could be asked to kill your wife, mother, kid, who knows? Man is too specific.
Two: You absolutely cannot know who it is or add any caveats, eg "If he was a bad man" or "if I didn't know him" --- that makes it too easy to make the decision.
Reminds me of this old "Twilight Zone" episode. Very similar (Tuna may have got the idea from it, maybe not)
A couple who are on hard times are visited by a mysterious stranger (people in the Twilight Zone are always being visited by mysterious strangers!) and given a box. There is nothing in the box but a button. They are told if they press the button, two things will happen: one, someone they do not know will die. Two, they'll be given fifty mill or whatever the figure was in the show --- it was a lot. Maybe a million (this was b/w TZ)
The man says he'll come back for the box in the morning. Whether they've pressed the button or not he'll take it away. The couple argue. The wife is all for pressing. What, she asks, if it was someone who's dying of cancer? The husband asks what if it's a newborn baby? Anyway they argue and in the end while he's at work she presses it.
He's angry when he comes home but there's nothing can be done. The next morning the guy arrives to take back the box. "I see you pressed it", he says knowingly, and gives them a fat wad of cash. The wife, still unable to believe this isn't some gimmick (which is what she thought it was, and didn't really believe anyone would die, and that's why she pressed the button) asks "And did someone die?" The man confirms they did, and that the couple did not know them.
As he's taking the box away, she asks what will he do with it now? He says it will be given to another client, with the same offer, the same conditions. His last words are chilling: "Don't worry", he says, "We will ensure the box is given to nobody you know!"
Anyone for changing their mind now?
I vote no, by the way: murder is not justifiable no matter how much cash you throw at me.