The Doctor is firm in his resolve, knows what he must do and is determined to follow through, whatever the cost. The fate of the whole universe depends on him; what’s one small planet compared to that?
Innocent people have already sacrificed their lives in order to buy him the time he needed to construct the Delta Wave, and now at last it’s finished. All he needs to do is activate the signal and every living creature within range will die, Daleks and humans alike.
What is he, coward or killer?
Resolution crumbles; he can’t do it.
“Coward, any day.”