Just look at this scene. We didn’t get to see this angle in the movie. 

Harley hasn’t woken up, and just look at Joker’s expression.

Right after admitting to himself that he needed her, he is faced with the notion that he might have been too late to save her.

Just moments ago Joker watched her free fall in to the chemical vat for him. The same vats that turned him into the monster he was.  He watched her disappear into the bubbling chemicals from the ledge and he had turned to leave, but a pain in his chest had caused him to stop. A growl resonated in his throat as he physically struggled with the thought of abandoning her. Feeling a need to save her.

He hadn’t meant to really care about her, but in that exact moment he couldn’t bring himself to leave… and he knew he was utterly fucked.

He had ripped off his jacket and dove off the edge, and followed her down. His hands found her body in the water, and he pulled her to the surface. Holding her limp body, as they emerged from the chemicals. Cradling Harley with a tenderness reserved only for her. Joker looked down on her porcelain face.

Motionless.

Joker felt a pang of dread. Everything came to a screeching halt, all the little gears in his head stopped turning. His chest felt heavy and his insides burned with the chemicals around him. Harley’s face remained still. Silence hung in the air. Just moments ago he realised he couldn’t be without her and now it occurs to him that he might have been too late to save her.