Bleed

On a cold night, two men are dressed up as a nun and a hooker. The man dressed as a hooker is brutally murdered, while his girlfriend (Julie Strain), who was waiting for him, is soon killed as well.
The story shifts to Shawn (Danny Wolske) at his new job. Maddy (Debbie Rochon) is interviewed by Shawn and gets the job. That night, while Maddy is having dinner at her apartment, she appears to be talking to imaginary people and seemingly receives a letter from her parents, whom she previously claimed were dead, congratulating her on her new job. When she goes to sleep, she dreams of two deaths and awakens in fear.
Shawn and Maddy get closer, go on a date, and end up having sex. In the morning Shawn invites Maddy to a pool party, to which she agrees. At the pool party we are introduced to Keith ( Barry Bedwell ), Karen ( Anne Star ) and the host Tillie (Orly Tepper), Maddy arrives and meets Shawn's other friends Chris (Allen Nabours), Peter (Ronny Gene Blevins) and Laura (Laura Nativo). Later, the remaining people at the party start talking about a "murder club". In confusion, Maddy leaves, disturbed by Chris's words to her: "You never know when you'll need the club's help". Karen and Keith are then seen having hardcore bondage sex when the killer enters the bedroom and strangles them. In her sleep, Maddy has a flashback of when she had an abortion. As a result, her mother (Brinke Stevens) had kicked her out of the house, calling Maddy a "murderer". Her father (Lloyd Kaufman) had tried to stop her, to no avail.
During one of Maddy's shopping trips, she starts a heated argument with a woman, eventually killing her by smashing her head against a large post. Maddy then awakes at home, covered in blood. She realizes what she has done and calls Shawn. He races over and tells her not to worry, as he'll give her an alibi. The next day, on her way to Tillie's house, Maddy hears Shawn, Tillie, Peter and Laura talking amongst each other. She then discovers that the "Murder Club" was all a joke. Shawn tries to convince the others to give Maddy an alibi. Chris agrees, and Peter and Laura want to ignore everything, though Tillie believes they should head to the police. Maddy heads home, upset by the revelation. She later has a dream of Peter and Laura being killed.
She visits Tillie's home again but hides to avoid being seen. Tillie talks to Shawn about finding Peter and Laura's bodies. Maddy then blacks out and has a vision of Tillie's death. When she awakes, she goes to Tillie's home and finds her dead. She goes to Chris's home, believing Shawn to be responsible for the deaths. Then she races over to Laura and Peter's home. There, she finds the dead bodies and tells Chris that she believes Shawn is behind the deaths. She heads back home and Shawn arrives. She attempts to attack him but he retaliates and tells Maddy that Chris is the real murderer. He shows a mask and knife, saying he found them in Chris's home. Just then, Chris kills him by impaling him with an axe. A shocked Maddy demands an explanation. Chris admits to the murders, saying that he did so he could have Maddy. The first death was the owner of the company. Killing him made Shawn head and gave Maddy a place to work. He then killed the various people who wanted to stop Maddy from being with him.
A small chase begins, with Maddy stuck in the bathroom of her home. Chris starts to chop down the door with his axe, but before he can reach Maddy, she hits him with a set of scales and flees. She then becomes trapped in the bedroom and the scene cuts to black. The next scene shows Chris heading to Las Vegas. He pulls over to a car stuck on the side of the road. The man inside asks for a tire-iron. Chris considers killing him, until he sees that the man has a wife. After helping them and watching the couple head off, Chris heads back to the trunk. He opens it, revealing a still-alive and tied-up Maddy. He smiles, then shuts the trunk, and resumes the journey to Vegas.

Question: Why did Mady leave the pool party?