Rose doesn’t know why anyone would fear this poor thing. Here it is, locked away in chains inside a cage it can’t possibly escape from. Helpless, suffering, repeatedly tortured, and she can’t help feeling sorry for it. Surely nothing deserves such cruel treatment.
It reminds her of the old salt and pepper pots her Gran used to have, with a few bits added on, what appears to be some sort of whisk, and a plunger sticking out. If anything it looks more comical than dangerous. All Rose wants to do is help it; isn’t that what the Doctor would do?