Exorcist question

This question has been bugging me for years. In The Exorcist, just after Regan ‘stabs’ herself with the crucifix and her head turns around, she says “Do you know what she did, your cunting daughter?”

So…what did Regan do? What is the demon talking about?

The demon’s voice when it says that line is that of Burke Dennings. Regan (the demon) was the one who killed him, and that’s what the demon is referring to.