I am by no means justifying Steven's actions. Steven is pure evil and had absolutely no redeeming qualities at all, and murder is not justifiable. Why, however, would you blackmail someone who would clearly resort to murder to protect and advance his career? How could she not see that she was putting her life on the line?
shareAgreed. If she thought Steven beat Roger's head to a pulp, she was taking a crazy risk by blackmailing this guy. And being intimate with him. She should have been very cautious around him and never allow herself to relax as she did.
The sad part is that she had her finger on the pulse; she knew a good sound. You'd think that would be a useful talent in an underling. Maybe that's why she took the risk. She thought she was indispensable in locating music and performing this service for Steven -- i.e. setting up the cop. But for Steven clearly much to risky to let her alone. From his point of view she had to die.
It makes much more sense in the book, where she is just crazy and wants to marry him. There's a lot more of an incentive for him to kill her in the book, not so much in the movie other than to frame the copper.
In the book there's a fleeting meeting with a character who wants to work in A&R, in the film she's merged into Rebecca.