Emperor Dalek: Then prove yourself, Doctor! What are you? Coward or killer? The Doctor: Coward, any day.
This is an important moment for the Ninth Doctor. It's the culmination of his absolute hatred of the Daleks, which has been built up over the course of the series. It's also the culmination of the guilt that Nine has been feeling about the Time War. It's when he realises that he can't make the same choice twice, he can't bring himself to destroy another race just to wipe out the Daleks. Eccleston is absolutely brilliant in the scene - one of his final few. It's obvious that the Doctor is frightened. He's scared out of his wits and beyond desperate. But once he comes to his decision, there's a note of defiance, as if the Doctor won't allow himself to be drawn into causing that amount of destruction again. Choosing to be a coward isn't cowardly here. In fact, it's brave, it's making a principled stand. At the same time, there's a note of resignation. The Doctor is accepting his demise at the hands of his enemies on his own terms, not expecting to be saved but greeting the inevitable with his own dignity. It makes the moment powerful and glues the climax of the finale together.