DOCTOR: Do you know what a gun is, Andarta? Do you know what a gun means? It only ever means one thing. One! Everything I've done, no matter how terrible, it has always made many other outcomes possible. Complicated, unpredictable events, things that carry life forwards. But a gun? A bullet? I can't do that.

(The War Doctor sets down a gun to not kill a human sacrifice that would have proven he's the legendary Warbringer)