Why did Bree sleep with Klute? and why did Peter kill himself?

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Early on in the film, Bree tells her psychiatrist that being a call girl and having sex made her feel good about herself, although it did not bring her any physical pleasure. As the previous poster said, at that point in the film Bree is in a very bad state of mind, so she has sex with Klute in order to feel good about herself and escape the horror, at least temporarilly. The thing is that having sex with Klute makes her feel good both mentally AND physicaly. Receiving physical pleasure from sex is something she is not used to and, as she later tells her phychiatrist, that scares her. So, calling Klute "John" after having sex is a defence mechanism, a way of emotionally detaching herself from the situation.

Now about Peter killing himself. When he confronts Bree, he says something along the lines of "...Does Klute know about me?... Then if Klute knows about me, everone knows about me...". So from that statement it is fairly obvious that Peter believes that the police know he is the killer and that whatever he does he will not get away with it (which is probably true, since Klute deduces who the killer is with the help of Detective Trask). Peter is quite crazy, so although his instinct tells him to kill Bree, when he sees Klute he probably realises that the "game" is over whether he kills Bree (and Klute even) or not, and decides to off himself instead.

Well, at least that's the way I see it...

Here is how I see it: Bree first slept with Klute to have power over him. She ACTED like she was afraid and needed him (which may have not been completely acting); remember that the only way she felt power over her life was manipulating men. That is also the reason she stabs him with the scissors.. while she didn't have complete love and attachment to Roy Schieder (the pimp), she still felt the need to attack Klute since Klute was attacking her meal ticket, as well as just the simple fact that, at that moment in time, Klute became an aggressive male, which led to Bree responding almost instinctively. I suspect Bree was maybe not sexually abused as a child, but definitely physically abused.

As far as "the killer" Peter falling backwards out the window, the impression I got was that Peter, when he saw Klute aiming the gun, realized he wouldn't be able to kill Bree (as he even said as much that he knew he was screwed, so Bree was a consolation prize before the inevitable would occur). Thus, he used her body as leverage to push from, so he could have one last moment of control over what little of his life was left. Remember that the entire theme of the movie was having control over one's life - Bree found power in manipulation of men; Klute found power in discovery of information; Peter found power in a more direct form; even the Pimp and the Psychiatrist found power in having people who came to them "to be sheltered under their wing".

This is also the reason why Bree didn't marry or settle down with Klute - to do so would mean acquiescing to her love for Klute, which is also the reason why she tells her psych why she hates that part that wants to push Klute away and destroy their relationship. She's in a Catch 22 - if she throws away the one man who truly accepts her, she realizes she will be alone for the rest of her life (which she truly fears. Look at Bree's face when the junkie wipes the brow of her fellow junkie boyfriend... the fact that even the junkie can find love and she CAN'T).. but if she ACCEPTS Klute, it would automatically mean being diminished in her own eyes (probably due to what happened to her in her childhood).

She was extremely attracted to him and afterwards discovered that she was in love with him. It scared her because she was never in love before and couldn't handle the vulnerability of it. She wanted Klute and was terrified that she would lose him or he would reject her.

So she insulted him to get a bit of distance.

Although the film is open ended, I like to think she settled down with Klute and won a bit of happiness in life.