OK Darvos is evil, but he's not wrong.
Yes the Doctor is anti gun peaceful resolution, but unlike US characters like MacGuyver, he doesn't always get to make the best argument.
Harriet Jones, Prime Minister (we know who you are) was right to blast the fleeing spaceship out of the sky in "Christmas Invasion". The bad guys or their associates will come back and Earth cannot rely on the Doctor showing up next time. And the show later showed what trouble the Doctor's hissy fit caused.
And even in the Classic series, the better companions (Leela, Ace, and even sweet, gentle Nyssa) often ignored the Docor's preaching and used a little violence to resolve matters.
Wow, I’ve GOT to see that eppie!
And yeah, Harriet Jones was right to destroy them...
Thanks,
Ed