Why didn't I like this movie?

And spare the ad hominems, please. Here's what I thought: I love film noir, even the ones that subvert the genre: Memento, Big Lebowsky, Kiss Me Deadly. And I love circa 1970s Elliot Gould. His Hawkeye in MASH is one of the most charismatic characters ever put on film. So Elliot Gould as Philip Marlowe in 1973 seemed like a perfect fit to me. But all it was was Gould wandering around mumbling to himself saying idiotic dialogue that nobody would ever say in real life. That's what I didn't like about it. You can tell the actors ad libbed the dialogue to the extent that it took me out of the movie. It was distracting. Also the plot was full of holes but I can forgive that. Anybody who's ever seen The Big Sleep or The Big Lebowsky will understand that the plot is often times secondary. And anybody watching a Robert Altman movie expecting a finely tuned narrative is missing the point. So I get it. To be honest, I was on the fence with the movie until the end. (SPOILER ALERT:) But at the end when Marlowe took out his "friend" with an act of cold-blooded murder, it also took me out--of the movie. That was the clincher. I may be wrong but this whole movie felt like the end result of Altman calling his buddies and saying: Hey, wanna get high, have some fun, and make an old-fashioned private eye movie? Elliot, you be Bogart and I'll be John Huston!