Scythe by Neal Shusterman
Scythe is a science fiction book set in the future where the human population is immortal. The only way to die is to be randomly killed or gleaned by professional reapers known as “scythes.” Citra and Rowan are two teenagers who have been selected to apprentice as scythes. They must learn the art of killing and understand the need. But, in a twist of fate, only one will be chosen to become a scythe and the winner will have to glean the loser.