Steven was understandably disappointed in Peridot when he found out he stole the Diamond line and still planned to contact Homeworld. And yet, she was on their side all along. Although she probably hadn't intended to lie to Yellow Diamond, she sensed that if she told her about the rebels, it wouldn't end well for the Crystal Gems. They'd been so nice to her after Steven asked them to give her a chance, when they had every right to keep her poofed and bubbled or even shatter her. She was loyal to Homeworld, to those the Crystal Gems fought against. They'd trusted her when she didn't deserve it, so how could she betray that? Check out more in the A.V. Club's review, at the link!