On the ABC medical drama, a plane crash claimed the lives of both Lexie Grey (Chyler Leigh) and Mark Sloan (Eric Dane).
"The death of Lexie Grey haunts me," Rhimes wrote in a post on her official blog. "I adored that character and I couldn't imagine the show without her.
"The plane was going to crash no matter what. I honestly wasn't sure, because of contract negotiations, who was going to die in that crash."
Rhimes added that Lexie could have died "by doing something as simple as slipping and hitting her head" but that the death in the crash guaranteed "that her exit would have maximum impact".
"She would have been dying over at Seattle Grace while other people were dying in the woods, which wasn't my favourite plan," Rhimes explained. "She would have died without Mark at her side. Without the chance to say goodbye to him. Without the chance to hear that he loved her. I wanted them together when she left us."
The showrunner described Sloan's own passing as the show's "most tragic" demise, insisting that she is "very attached" to the character.
"He's part of the fabric of the show," she claimed. "Before he ever even appeared on the show, the characters discussed him. He is one of the originals."
Of his death, she wrote: "I fought it and I debated it and tossed and turned about it but in the end, I had to do what was right for the integrity of the character.
"Mark would never abandon willingly Sofia and he would never willingly abandon Callie. So Mark dies. And he and Lexie get to be together in a way. Their love remains true."
Grey's Anatomy airs on Thursdays at 9/8c on ABC. In the UK, the series airs on Sky Living.