*Could* you sell your soul for the salvation of all mankind?
Assume that there is at least one person you would sacrifice yourself for; as they are in the subset of all mankind, you might as well save a few others for your trouble, as it were. But wouldn’t they for you, being someone who’d them? Either by agreeing to the same sacrifice or ironically to spare you from such a decision, the devil would O. Henrily have you 2 sacrificing yourselves mutually, saving the complement you didn’t care enough about in order to inadvertently damn each other. Extrapolated to all of mankind: the part that would be worth salvaging would agree, so that all of mankind would sell their souls to the devil, and what remained could not be called humane. That’s the devil's way: sell your soul to save humanity, and discover that by doing so, it is lost. Religion and logic in agreement; selflessness gets martyred.