Barclays’s purchase of Lehman Brothers Holdings’ brokerage in the falling markets of September 2008 occurred in “the riskiest week of my life,” the UK bank’s chief executive said. “Markets were collapsing,” Barclays group chief executive John Varley told a federal judge today in Manhattan. “We needed to ensure there was a buffer between assets and liabilities. We needed to be able to look regulators in the eye and say the deal would be capital accretive.”