Renegades by Marissa Meyer
The Renegades are human, but with extraordinary abilities. They emerged from the ruins of a crumbled society and established peace and order where chaos reigned. As champions of justice, they remain a symbol of hope and courage to everyone... except the villains they once overthrew. Nova has a reason to hate the Renegades, and is on a mission for vengeance. Adrian is a Renegade boy who believes in justice-- and in Nova. But Nova's allegiance is to a villain who has the power to end them both.
Review from Booklist Reviews:
Nova Artino believed the "good guy" superheroes, known as the Renegades, would save her family from an unknown assassin; but at the end of the day, her parents and younger sister are dead, and she's no longer a fan. Her uncle, a villain in this futuristic society with a good number of supernaturally talented people, takes her in, and Nova discovers her own talent, becoming Nightmare. Her goal: to infiltrate the Renegades and bring them down. She just doesn't count on a surprising reluctance to kill people. This series opener establishes the central question of what makes someone good or bad, and whether the distinction is just a matter of perspective. The Renegades represent the rule of law, but their methods are sometimes suspect, while the anarchists see a society weakened by dependence on superheroes. It is a timely, thought-provoking, discussion-worthy premise wrapped in a narrative bogged down in too much description and explanation-though the author's skills are apparent.