Hammerhead's the enforcer. It's her job to identify threats and guard them all from them, however she can. As it is she has no reason to believe Jack isn't a threat. Every personality in the Underground had access to the memory of Jack's first post in Dualis, the way she'd raked her fingernails across the raw flesh of a barely-healing wound by insinuating they'd been too stupid to save themselves but she could if they did what she wanted.

The Hangman's Daughter wasn't scared of her, and even Jane doubted she was as bad as they'd first judged, but she was as unnecessarily cruel as a slap on a sunburn and this dream had made Hammerhead re-evaluate things. She had no real reason to trust Jack, but as she watches her laugh at them (again), she tenses up again, curling her lip in disgust.

"You're right, you're not. And by the way, FUCK YOU AND FUCK YOUR FILE YOU PISSY LITTLE BITCH!" She turns immediately towards Murmur, intending to disregard any protest Jack might make.

"Why the fuck should I trust you?" she growls, hands curled into fists at her sides.