Sniper (Vintage Paperback) by Theodore Taylor

When Ben's parents go to Africa, they leave the fourteen-year-old in charge of the family's wild animal preserve. Everything seems to be running smoothly until one night when the silence is broken by the sound of peacocks screeching. When Ben leaves the house to investigate, he sees a terrifying two lions shot dead from bullets sent straight to their hearts. Someone is out there, someone with a score to settle . . . and there's no telling who will be the next victim.