Isabella was what you'd call a hopeless romantic. When a girl like her falls in love, boy does she fall hard. Justin was quite the opposite. He hated the mere idea of love: how it made you so weak, able to be easily manipulated.
When Isabella professes her love for Justin, things go south. He does not want to love anyone, nor does he want to be loved. Justin becomes hell bent on changing her feelings, and decides that abuse is his best option.
Maybe it's because Justin has had a rough past when dealing with love, or maybe he's simply psychotic. Isabella decides either way, she's not leaving the man she loves, nor will she ever stop feeling this way about him. Her only option is to find the heart buried deep inside Justin (if there even is one), or else she may be doomed to a life of pain and extreme heartbreak.