I think Graves’ motivations are a combination of anger at Gellert, starting to see Credence as less deserving of pain just for existing, and being just a little bit moved by Credence offering himself up for all these awful things just to make him feel better – but of course it’s Gellert who does those things, so Graves does the opposite.
I don’t think Graves sees himself as caring for/about Credence, so if he were to reflect on that moment he’d probably bring it back to his anger and frustration with Gellert. But just because Graves won’t admit it, doesn’t mean that’s all there is to it.
And as it happens I’m also really invested in this series, and will take any excuse to talk about it 😀