The movie shows us that no matter what nobody can make a perfect crime. It starts with a murder.
The main actress is black and the killer. There are no suspicions about her in the whole movie. All the clues show that the murderer is another person (I think it's a man, not sure). That person is accused and punished for this crime although he/she is innocent.
I am sure about the final scene. Someone realises and is able to prove who the real killer is and talks to the killer in an apartment. After that someone reveals that to the guilty woman, she stabs him (not sure again about the sex of "someone") to death. but the camera turns to the window of the apartment opposite the apartment of this murder and we see that the whole scene has been seen by a witness (white man) and then the movie ends.
I thought that the main actress was Halle Berry but the online search denied this, don't know if that helps.