Rufus was distracted. His headache was awful.... he had to connect to Mac2, which was getting more demanding. Seeing Amy on the doorstep when expecting a grocery delivery threw him. For a moment he couldn't process it. He was confused.
'How had she found him? What was she doing? Hadn't she done enough! Bitch!!! Shit, I must get rid of this headache, then her.'
His choice of attending to his headache first saved Amy's life.
Amy, a good judge of character, could read people well. The hate she saw in Rufus's eyes screamed 'flee!'
She ran. Rufus couldn't possibly follow.