How can I make these characters more exciting?

I'm writing a short novel about a young girl of 19/20 years old, who falls in love with a man of about 26. She leaves home to be with him because her family think he's trouble. She drops everything for him. A year or so down the line she's living with him and his brother and his brothers girlfriend. Her partner and his brother are involved in a lot of dangerous stuff. They've killed many people and d young girls, etc. The girl is expected to keep her mouth shut and cover up for them. Being naive and thinking she's in love, she does as she's told. But eventually she gets caught up in one of the murders. She doesn't actually do anything, but she's forced to watch a teenage girl being brutally killed by the man she loves and his brother. She knows then that she has to get out but she's petrified. In the end she gets away, but it's a long story. I'd just like to know how to make the characters more interesting and gain some sort of understanding on the girls situation. Her boyfriend is a murderer so I'm going to make him a bit abusive towards her. How would she act and how would he act, etc.? It'd be a great help if anyone with an understanding could elaborate.