AS A teenager, Travis Kalanick’s first job was to knock on strangers’ doors and sell them knives. Now he is trying to dodge the daggers aimed at him and at Uber, a ride-hailing firm that is the world’s most valuable startup. On March 19th Jeff Jones, the company’s president, stepped down after six months, declaring that “the beliefs and approach to leadership that have guided my career are inconsistent with what I saw and experienced at Uber.” At least six key executives and high-ranking employees have left in the past nine weeks.