Boom Town by Russell T Davies

Script Quip:

MARGARET
I promise you, I've changed since we last met, Doctor. There was this girl... just today... young thing. Something of a danger. She was getting too close. I felt the blood lust rising, just as the family taught me, I was going to kill her without a thought. And then... I stopped. She's alive somewhere right now, she's walking around this city because I CAN change - I DID change. I know I can't prove it--

THE DOCTOR
(calmly)
I believe you.

MARGARET
Then you know I'm capable of better.

THE DOCTOR
It doesn't mean anything.

MARGARET
I spared her life.

THE DOCTOR
You let one of them go, but that's nothing new. Every now and then, a little victim's spared. Because she smiled... because he's got freckles... 'cos they begged... and that's how you live with yourself. That's how you slaughter millions. Because once in a while, on a whim, if the wind's in the right direction... you happen to be kind.

MARGARET
(coldly)
Only a killer would know that.

Pause. Not the answer the Doctor was expecting, and he's thrown.

MARGARET (CONT'D)
Is that right? From what I've seen, your happy-go-lucky little life leaves devastation in its wake. Always moving on 'cos you dare not look back. Playing with so many peoples lives - you might as well be a God.

The Doctor loses eye contact, slightly hurt.

MARGARET (CONT'D)
And you're right, Doctor... you're absolutely right. Sometimes... you let one go.