6. Gasses The Silurians In "Warriors of the Deep"
In this so-so base under siege story from Peter Davison's final season, The Doctor is forced into a corner by the Silurian's plan to launch a nuclear war, rendering the Earth uninhabitable (wait, don't they want to take over the Earth in "The Silurians"?) and uses some conveniently placed hexachromite gas to poison them, reducing them all to steaming piles of bubbling green goo. As with most stories of this era, the body count is exceedingly high here, leaving only four characters left alive by the end (The Doctor, Tegan, Turlough and senior base officer Bulic). Here, the Doc makes basically the same decision as the Brigadier in "The Silurians", showing his earlier offense at his actions to be somewhat hypocritical. Unlike the Brigadier's decision to slaughter a whole race for reasons of simple expediency, The Doctor is lead to this point by his own hesitation and indecision, trademark traits of the rather naive 5th doctor. It's also somewhat hypocritical that in this story, the Security Chief for the base insists he use the gas to stop the Silurian plot, which the Doctor initially refuses to do, accusing the Chief of advocating genocide. Even though he ultimately saves the planet (again), he realizes the ultimate emptiness of his victory as he ruefully mutters, "There should have been another way."