Eight

Not an actual 8th Doctor quote in case you were wondering.

This one, however, is:

“I used to be that guy. A man with a master plan, seeking out injustices and toppling governments all in the name of the the greater good. I'd started to do the maths - that's how evil starts, with the ends justifying the means. Once you start down that road there's no turning back; what if you could save a million lives but you have to let ten people die - or a hundred, or a hundred thousand? Where do you stop? .... That's why I no longer travel alone, so I could never forget how precious a single life is.”

So yeah, remember that bit in the anniversary special when they decided NOT to blow up Gallifrey? That was Eight.