Why is that tied to some notion of an afterlife? If anything, it seems to me that the reality that this one life is all any of us have should make one care way more for their fragile peers, and for making it the one life they have count. As The Doors once put it, "no eternal reward will forgive us now for wasting the dawn".
It doesn't have to be. But it does indicate accountability, and some people do hope get away without it.
I can't think of a more important idea for a conscious living being than some hope for an afterlife, perhaps a better one than is here.
Most people resorting to and flood the discussion with rational X or Y are simply getting high off of their own sounds and are plain dishonest with themselves and others.