How we cite our quotes: (Part.Chapter.Paragraph)
Quote #7
[Clary] ran a little way up the hill and shouted [Simon's] name. […] She called out to him again, but he didn't turn around. (2.8.421)
Simon gets betrayed by Clary a lot in the this book, but seeing how much she enjoys kissing Jace, her own brother, is the straw that breaks the soon-to-be-vampire's back.
Quote #8
"I saw the look on his face before he took off," Jace said. "You didn't. You weren't looking at him. But I was." (2.9.24)
It's telling that Jace, who hates Simon, is paying more attention to Simon than Clary does. She takes Simon for granted so much that she sometimes forgets he exists.
Quote #9
"Valentine needed a new second in command and he wanted Stephen. […] Then Stephen was killed in a Circle raid on a vampire nest. Celine killed herself when she found out. She was eight months pregnant at the time. And Stephen's father died, too, of heartbreak. So that was Imogen's whole family, all gone. (3.17.66)
This background information helps us understand how the Inquisitor was betrayed by Valentine, but honestly, it sounds like her son betrayed her more than Valentine did.