Black Cat in Jail (2000)

The film opens with Jade Leung kills a few people - but when she accidentally threaten a young boy in the process in a square where she tries to save him, thereby allowing sufficient time for the police to arrest her and send her to imprisonment. There is a focus not only on her, she shares it with three other prisoners as she slowly befriends, and each of them given her own story and flashback on how she got there.