They seemed to know that Eric wasn't backing up because he was afraid of them, but because of the woman he looked toward. Leeloo stepped through the remaining muggers as they turned toward her - turned, but made no move - and she set her back to Eric, instead facing them.
"Run," she said. When they didn't move, she shouted it at them. That got them going. Some even dropped their weapons in their haste to get away from the pair on the street. None of them wanted to die, and death had come to this alley.
When they were gone, Leeloo turned around and looked at Eric. She didn't know what to say. She'd never seen him kill before. It went against everything programmed inside herself, and she struggled to reconcile the killer in front of her with the gentle man who'd kissed her to sleep every night since they redecorated his apartment.