No light reached the cell of Collei. It was dark and hollow.

You didn't know what you signed up for, taking a minimum wage job under Il Dottore. You knew that the Fatui could be cruel but you never expected that they would experiment on a literal child. It was gross and horrifying.

She despised the world, humanity, the archons, but the Fatui most of all

You couldn't help but want to aid her.....even if it was considered treason.