Anna keeps sneaking glances at the blood. There's so much of it, and she has to fight hard to even hold on the the scraps of a smile that she's trying to desperately to maintain. It's really, really not working. Eventually she gives up and drops her hand. Wouldn't it be so much easier to walk away, maybe try and survive her way back to camp with no ammo and a hunting knife as her only defence. Slim chances there.
It'd be easier, sure, but there's not a chance in hell she's going to do it. Instead she moves closer, arm quickly wrapping around the backs of his shoulders. She'll take some of his weight, she can do that much, and her free hand gently moves one of his away, replaces the pressure. This at least she can offer, as little is. Light hearted conversation, too. Gallow's humour. She can give him this.
Except for the part where she finally makes eye contact and it all sort of bursts out before she can stop herself.
"Crap, Cas I'm so sorry. I should've gotten there quicker, I could've-"
Could've what, exactly? Stopped this? Unlikely. Anna's always been good at feeling unearned guilt though, this isn't any different.