About halfway through the three-hour siege at Trader Joe’s in Silver Lake, the wounded gunman, Gene Atkins, looked at one of his hostages, MaryLinda Moss, and told her it was all over for him. “I just shot at a cop,” he said. Moss, a 55-year-old artist who exudes calm, feared a suicidal gunman could spark a bloodbath. Through a series of disastrous decisions by Atkins, dozens of strangers had ended up at the grocery store on a hot Saturday afternoon, drenched in fear and surrounded by SWAT teams, helicopters, squad cars and ambulances.