Worst Case by James Patterson & Michael Ledwidge

A calculating killer is abducting the children of New York City’s wealthiest families—but ransom money isn’t what he wants. Instead, each victim is forced to answer questions about privilege, wealth, and responsibility, with deadly consequences for the wrong answers.
 
Detective Michael Bennett is assigned to the case and joins forces with FBI agent Emily Parker as the investigation becomes a race to stop a killer whose disturbing plan is unlike anything Bennett has encountered before.