The script gives obvious clues. At the end, if the girlfriend ran to her boyfriend's house and wanted to cheat on her boyfriend, the reasonable design was that the boyfriend only had to shirk the responsibility, but not only did he not appear to be a control freak, but he also said some strange words, implying to the audience that the girlfriend was also very disrespectful. It's normal, and he understands the abnormality of his girlfriend and loves it, so his control freak is because he "Deserves" his girlfriend this way, so he wants to do it all over again.
As a result, his girlfriend killed him, and not only did he ghost mannequin effect avoid the monitor to show the evil expression of premeditated murder, but also gave the expression of a serial killer in the face of questioning by the black police. In the silent picture of the final ending, the girlfriend even showed the look of "I am seriously crazy". This means that from the beginning, the girlfriend may be a psychopath that is far more abnormal than her boyfriend and her boyfriend's brother. Otherwise, according to the traditional routine, the girlfriend should have an ordinary attitude of revenge or "Relieved because the threat is removed". But she didn't.
It is also possible that the girlfriend was originally a normal person. After going through all kinds of things, she was influenced by her boyfriend and her boyfriend's brother, and the result became abnormal. So at the end, the black policeman showed a cautious and fearful expression to her, which is very accurate. Because the expression implied that in these possibilities, he couldn't figure out what the girlfriend really thought, so he could only pretend to be okay, lest he and his daughter would also be implicated.