why did rooster rape karen?

I also didn't get this point - it seemed totally out of character.

The only weak reason for the plot point (not explaining the character's motivation) is that Rooster's rape of Karen brings Karen to Spider's place with a gun, which is ultimately used to kill Rooster. They could have easily deleted the rape scene and had Turk find a gun on Spider's body, though.

I was disappointed by this film. The plot twist was very obvious from the very start of the movie, the dialogue was weak and the lead actors unbelievable in their roles.

To do something unforgivable... or some sh!t.

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I think like the shrink said, he wanted to be caught. The police are getting real close on catching the 'poetry boy'.
At the last scene, even if he wanted to run away, he knew that he'll be caught eventually. And if he's caught or shot dead, I think Rooster wanted the person who do that was Turk himself.

I thought he did it to enrage Turk enough to kill him--suicide by cop/partner. However, unless I missed it, the film doesn't show Turk finding out about the rape. Turk only shoots Rooster in self defense. Overall, I did like this movie, but I find the rape puzzling and out of character if it wasn't done for any reason.

Nobody gets to be a cowboy forever.

Since the Russian survived, Rooster knew he would be identified therefore ending his secret killing spree. It's not as fun when everybody knows who commits the murders so he wanted to do something extreme for his last hour of being the poetry killer. And it was obvious he wanted a piece of Karen early on in the film like when he says "You're a bad little girl" among other obvious allusions-hell, Turk TOLD him to have sex with her at the bar. Again, Pacino and DeNiro were GREAT in this movie.

But of course that was only a fringe benefit to his diabolic plan (cue sinister music). The whole point of the film is based on a twisted rivalry with his role model and longtime partner who let him down during the Randall case (Rooster found this "unforgivable"-see how that works?). He was finally making a real difference by stepping outside the law to take out the trash therefore making him superior in his mind. Making Turk read his confession in front of the camera was another way of display of power and while we're at it, it's worth repeating that raping his girlfriend Karen was the icing on the cake to prove he was the better man because it's no secret she liked it rough.

Edit: And I don't buy this business of Rooster wanting to get caught or having Turk taking him out. He wanted to escape after killing Turk himself (Heat anyone?) but as we saw during the final confrontation, Turk was the better shot kinda spoiling Rooster's plans. Had he escaped and disappeared I could see Dennehy screaming "I just had coffee with Fisk A HALF HOUR AGO!"

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I think its because Rooster doesn't like things that are evil. Hence why he lost it when Turk did something that isn't right morally and he went crazy and start killing the evil bad guys. Now Karen's promiscuous behavior can be deemed morally wrong don't you think? Yeah its her life, but its also playing with emotions. Although I dunno if roosters was trying to get something with her earlier, maybe to test her in a way just to see what a bitch she is :> At least thats my take of it. Morally wrong can sometimes piss people off and it probably break Rooster's limit point