Dally and Lia's relationship was purely physical... For Dally. Lia started to feel something deeper for him and became incredibly angry when he didn't return that deeper feeling. That's part of the reason she became abusive. She said the first "I love you" and when he didn't say it back, she was humiliated, hurt and angry, so that's when she struck the first blow. After a while of escalated abuse, Dally finally left her.
