Scythe by Neal Shusterman

What if death was the only thing left to control?

In the not-too-distant future, humans have cured hunger, disease, war, crime – even death.

Now scythes are the only ones who can end a life – and they are commanded to do so to keep the size of the population under control. Citra and Rowan are both chosen to be apprentices to a scythe – something neither of them wants. Forced together, they must perfect the skill of taking a life, competing against each other for just one scythe position.

But then it becomes clear that the winning apprentice's first task will be to kill the loser...