Bella didn't volunteer.
The drugs make things hazy, but she remembers this clearly: she never wanted to stay behind, to monitor this house of horrors her employers built. When whispers became susurrations became screams, when patients became corpses and monsters roamed the halls free, she was all for barricading the doors and leaving the facility buried. There’s nothing here worth saving, no knowledge worth documenting and disseminating for later experiments. If asked, she'd have told them to burn the subjects, their notes, the whole damned building. She’d nuke it all.
But they didn't ask Bella; after all, Bella didn't volunteer.