Scythe by Neal Shusterman

The premise of this book is interesting; set in a future when humans have eliminated death by wars, disease, age and crime, it has become necessary to cull the population through the use of Scythes; specially trained and appointed humans who are basically responsible for gleaning (selecting and killing) humans to keep the population down. CItra and Rowan are chosen to apprentice to a scythe, and neither of them are eager to take on the job. They are even less thrilled when they find out that only one of them can become a scythe and the successful candidate will have to glean the other.