Marquis' view is that depriving a being of a future like ours is a sufficient condition of the wrongness of killing. In other words, it is enough to make killing wrong. ON his view there need not be anything else that makes killing someone wrong. If by killing a person you deprive her of a future like ours that's enough to make killing her wrong.
Marquis also holds that depriving a being of a future like ours is not a necessary condition of the wrongness of killing. Depriving a person of a future like ours is not required to make a killing wrong. He proposes that perhaps there are other things that make killing someone wrong, such as the taking of another person's life causes grief and suffering to that persons loved ones.
It should be understood, in addition, Marquis is not, therefore, saying that it is morally permissible to kill anyone who does not have a future like ours. For example, Marquis would argue that the infant born with anencephaly did not have a future like ours, but he could still say that it would have been immoral for a stranger to sneak into her hospital room and kill her against her parents' wishes.
