I'm going with accidental suicide.
You've seen Guy Standeven in something because the man was in everything.

He threw himself out of the window.
Its that man again!!

Thrown.he goes through the window backwards with his arms up like he was defending himself. Who jumps through a window backwards?
"Remember, you have to make it home to get paid" (The Dogs of War)

If that's the case, he must have turned around, because I saw him fall out backwards doing a high kick.
See that? I posted those four hearts just for the hell of it!!
"There is no escape, John!"

I knew I shouldn't have deleted this before I had read the board.
I would say suicide, and I would say he was running forwards through the window not backwards.

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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".

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