In the Doctor’s vast experience, urban legends usually contain at least a grain of truth. Something nasty lurking in the sewers? Could be an alligator, or a mutant turtle, or maybe just an alien stranded far from home and making the best of a bad situation.
Then there’s the figure in the mirror, the one you catch a glimpse of and think someone’s standing behind you. They’re not.
The Doctor avoids mirrors now, except for once a year. He’s not without compassion; perhaps someday he’ll forgive her. She’s only a child.
Or perhaps she’ll remain where he trapped her. Forever.