Ruby didn't know what she thought would happen, as strong, as powerful as she'd made Sam, he couldn't stand against Castiel. He didn't have a chance and yet he still pushed it.
The Boy King had one purpose, free him, and then use the power to help him, find a place in the new world he'd create, demons ready to line up behind him.
"...Its over Sam, its done" she said simply from her place in the trap. She knew she'd lost, but still she couldn't bring herself, mostly out of a selfish fear of John and what he might do, to tell him the whole reason, her actual plans for him.