Even though I have always been pro-Pam, I totally think Pam ruined Katherine’s life. Pam actually couldn’t understand why Katherine was enraged at Cliff! He ruined Katherine’s father’s company, he got Rebecca killed, etc., and Pam is totally dense about the whole thing. Katherine has every right to never want to see Cliff or Pam ever again!
I'm amazed that people say things like "Cliff got Rebecca killed" or "It was JR's fault that Rebecca died." I'd like to know this: Who forced Rebecca to get on that plane? Did Rebecca not have free will? She was on her way to get involved in a business deal that wasn't even her deal; she had no need whatsoever to go!
She got on that plane because of her own vengeful, sadistic nature. This is a woman who once said: "Since the only way to your heart is through your company, I plan to do everything in my power to make that company bleed." This is the same woman who abandoned a 5 year old son and an infant daughter and left them in the care of an alcoholic. When she later became wealthy she never even went looking for them. She allowed her own children to believe she was dead. Pam came looking for her and when Pam found her, Rebecca looked her right in the eyes and said that she wasn't her mother, that Pam was mistaken. Pam couldn't understand why Cliff wanted to have nothing to do with this woman!
Rebecca was a nosy, vengeful woman who chose to get on that plane of her own free will. She was on her way to "make that company bleed." That's the type of hateful pursuit she chose to get involved in. You live by the sword, you die by the sword.
Cliff didn't make her get on that plane and JR didn't even want her on that plane. Rebecca wanted to be on that plane, so if you're looking for motive, that's it. That is the reason Rebecca was on that plane. If you're looking for the cause of her death, it was pilot error, pure and simple as it sounds. One plane collided with another and that's why Rebecca died.