Doctor Who: Every Doctor's Darkest Moment
6. The Tenth Doctor - The Time Lord Victorious
The moment at the end of The Waters of Mars where Adelaide Brook steps inside her house and shoots herself in the head is one of the darkest scenes in all of Doctor Who.
So of course the Time Lord Victorious is the Tenth Doctor's darkest moment.
It reveals how angry and lost the Doctor is without Donna Noble (or without a companion in general), and demonstrates the full extent of his "powers" as a lord of time.
If it weren't for Adelaide, the Tenth Doctor could have gone on to rampage through the vortex, bending reality to his own sense of morality and justice.
Despite nipping this in the bud fairly quickly, the moment in The End of Time where the Tenth Doctor rages at Wilf because he's capable of "SO MUCH MORE" is a worrying flash of the Time Lord Victorious underneath the surface.