The pull on her arm hurt, she fell to her knees as he cut into her arm, gripping it tightly, She found herself wincing for the first time since they'd started this, It was still nothing like the pain she'd endured in the Pit, from Alastair, but it was unusual from Sam, he was desperate, maybe mad at her for something beyond her control.
So she just went with it, let him drink, let him get his fill, he was always calmer afterward, more rational.
"Its okay Sammy, take what you need, don't worry...just take whatever you need"
Calm him, she had to calm him, she didn't want him deciding he didn't want it anymore, she'd seen that look in his eyes, and of course if it left his ststem he wouldn't need her anymore, might even hate her and then she'd never convince him to trust her enough to get him home. Kill Lilith, free Lucifer.