During the Time War, the Time Lords became among the worst civilisations in the universe.
His snatching the gun is already a way to show us how terrible the Time Lords must be. The Doctor also explains this to Wilfred in the same episode, why the prospect of their return filled him with such dread:
WILF: But I've heard you talk about your people like they're wonderful.
DOCTOR: That's how I choose to remember them, the Time Lords of old. But then they went to war. An endless war, and it changed them right to the core. You've seen my enemies, Wilf. The Time Lords are more dangerous than any of them.
-- "The End of Time, Part 1" (emphasis mine)
It's worth noting that even though they were changed by the Time War, the portrayal of Time Lord society in the show has never been particularly rosy. In their first ever appearance, they put the Second Doctor on trial for the crime of helping people, find him guilty, and sentence him to forced regeneration and confinement on Earth. The Sixth Doctor also gives them this contemptuous speech:
DOCTOR: In all my travellings throughout the universe I have battled against evil, against power-mad conspirators. I should have stayed here. The oldest civilisation, decadent, degenerate and rotten to the core. Ha! Power-mad conspirators, Daleks, Sontarans, Cybermen, they're still in the nursery compared to us. Ten million years of absolute power, that's what it takes to be really corrupt.
-- "The Ultimate Foe" (emphasis mine)
The entire planet and civilisation of Gallifrey is far more dangerous than one psycho.
The Master is psychotic, yes, and he's essentially destroyed humanity by grafting himself onto everyone. But the Time Lords are (now) an entire civilisation of psychotic war criminals, with all the power of Gallifreyan society, which is said to be the most advanced in the universe. They could do far worse than one single Time Lord could. And indeed, they do try to do far worse: "the end of time itself" (whatever that means) will affect a lot more than just one planet, no matter how much the Doctor cares about that planet.