Compare this to the inhumanity of life imprisonment and and the cost to the public of keeping a serial killer. Apply the greatest happiness of the greatest number. Some have a freudian death-wish and go out of their way to enhance the risk and the thrill. Mackie suggests that what matters is not so many deaths but the diversity and richness of living fuelled by appropriate dispositions. The risks become acceptable whereas deliberate killing is not.
People know the dangers and take risks - only with their own lives and for known benefits.
But they are nameless and cannot be isolated. Their concern becomes abstracted. So an ethical solution involves a compromise between those who want risk and those who do not and the resolution of self-referential altruism. I may want to commit suicide but others may have claims that tell against me doing so. This is controversy requires intelligent public discussion and awareness of the constituent values, rights, and dispositions.