Scythe by Neal Shusterman

Two teens are forced to murder—maybe each other—in the first in a chilling new series from Neal Shusterman, author of the New York Times bestselling Unwind dystology.
A world with no hunger, no disease, no war, no misery: humanity has conquered all those things, and has even conquered death. Now Scythes are the only ones who can end life—and they are commanded to do so, in order to keep the size of the population under control.

Citra and Rowan are chosen to apprentice to a scythe—a role that neither wants. These teens must master the "art" of taking life, knowing that the consequence of failure could mean losing their own.

Scythe is the first novel of a thrilling new series by National Book Award–winning author Neal Shusterman in which Citra and Rowan learn that a perfect world comes only with a heavy price.