The real story is
Unni(Fazil) and Alex are friends and jointly enjoying murderers act inside the bungalow. Alex brings Archana to the bungalow as if he is a stranger to the place. But, when he found Unni was inside his bungalow he never got surprised and it went on smoothly till they did a drama as if they were strangers to each other.
Alex never expected his friend to be there in the bungalow because he never brought Archana as the victim. He would have really loved Archana to be her future wife rather than to kill her. And Alex was playing subtlety and wanted to protect her from his friend unni. That’s why when matters went serious he was never allowed to call police or kill unni. Even unni says to archana that Alex will never kill him but brought her here to get killed. When Unni tries to reveal the secret to archana, Alex nods to Unni not to tell.
Unni never attempts to kill Alex or Alex (even though he has a gun) never attempts to shoot Unni.
In the climax, Archana understands that Alex was behind the murder and he tries to cover-up the killing which is happening in the very basement of his house. She never made any attempt to escape coz escaping means she will be near to her death.
Note : If you watch the movie both Alex and Unni are comfortable in exchanging and continuing the conversation. It may look strange but there is also another ANGLE where Unni would have disappeared long before and appeared on that night to steal 5 engagement rings for his survival. Archana understands everything in the climax that Alex is part of happening and she is shocked Alex is carrying his mobile, then it adds like preventing to call police, preventing Unni from talking to her, pretending as if he never had any connection to the killing. Pretending he never brought her to murder, Anxiety in searching for a jewel box which contains 5 engagement rings etc etc….
Finally,
During conversation Unni says 6 girls thinking that he has brought her as a 6th victim but Alex will reply 5. It means he has never brought her up as a victim…
Superb Movie!