Is that a warning? Or is it an observation about teamwork? Of course, John might interpret it as a warning because he knows that he's pretending to be friendly with Will. Or, at least— there was some genuine sympathy, at first, for Will's situation. It's not easy to end up here suddenly. But the comment about hunting monsters sheds a bit more context onto his comment about not all the blood being his. But it could also just be a comment about teamwork. Even if it was a warning it would be suspicious to just turn Will away here, and John has never balked from a challenge. He's not going to start now.

"Or maybe, someone is an ally waiting to be made. It can be hard to go at it all alone." Not that John has taken this advice, though he's well aware it is advice. He's not exactly alone either, he has Harold. And that's enough. Everyone else can just be a card for him to play when the time is right. That's why he's gathered them.