Duo really wasn't trying to get her to stop fighting. Just start taking care of herself. That was his only issue here.

"That's all fine and dandy... But you're forgetting something." Duo took a step closer. "The road to hell is paved with good intentions and you can't save people when you're dead. And if you keep doing what you're doing that's gonna happen..." and that scared him. How the hell dare she make him care for her, make him consider her a friend, and then go and just throw her life away without a care in the world?