Bruce doesn't flinch. He could because hearing that is a blow; he can take people calling him names, he can take 'beast' even, but 'monster' is always going to be too much, to hit too hard, down deep where it really hurts. This isn't even about the Hulk; it goes way back to his father who treated him like an object, nothing better than an unwanted stray dog. He doesn't flinch because if Loki means it on purpose, he's just talking about the Hulk, which is the surface level of Bruce's issues.

It takes effort to stay still though, so maybe he betrays himself anyway, and the hurt fills up his eyes slowly before he looks away, sniffing casually like nothing happened.

"That's not true. You can fight it." He lifts his head and the conviction in his eyes is old. "They call you a monster; that doesn't mean you are."