To get a heart you must gkill the girl when she is at the bench. Ive seen a video of it, but the guy used savestates, wich makes the timeline too complicated to understand. You talk to her, and i guess you do some other stuff every day, untill she asks you to enjoy the sunset on the mountain, and you kill here there, without going on the bench, as that end the game.

You have to give her all 10 flowers that can be found through the map. She'll then give you a special flower (for the ritual), and in the third day she'll invite you to see the sunset. Kill her like the other dude said.