From Daniel Klaidman’s book, Kill or Capture: The War On Terror And The Soul Of The Obama Presidency, a conversation between candidate Obama and former counterterrorism adviser Richard Clarke:
Looking the senator in the eye, Clarke stated a simple fact. “As president, you kill people.” He wasn’t just talking about sending troops into battle—in the shadow wars, presidents know the names and addesses of people they have killed. Obama stared back at Clarke for several seconds. “I know that,” he said very quietly and calmly. ” He didn’t flinch,” Clarke later recalled.