Guilt
Walt sees her.
To say Walt can live with his sins is the biggest lie he will ever tell himself every time he gets up in the morning. Walt has done many things in his life since the cancer; made hard cash by ruining lives and marketing off their stupidity and ignorance. He'd torn apart his family with his secrets and drove away his loving wife and son. He'd destroyed lives and took them away with brute and calculated force. He manipulated, coddled, and mind-fucked Jesse Pinkman with reckless abandon and caused him countless heartaches and disappointments.
He coldly murdered Jesse's junkie girlfriend and let Jesse take the blame.
Ghosts of his victims come to haunt him everyday, their gaunt faces and empty eyes wear Walt's conscience so thinly it's a wonder if he has one at all. But there, among those faces, is pitch black hair, snooty button nose, judging green eyes...
And vomit-tinted lips.
She is always there, eyes bitter as she watches him.
How do you sleep at night, she asks.
Walt honestly doesn't know the answer. And he doubt he ever will.